I read...a surprisingly large amount of that.

If I understand it right, they are saying that modern scholarship confirms that the Gospels avoid certain obvious failure modes - eg being written hundreds of years after the fact, wildly contradicting each other on important points, and erring on simple points of geography and history - and that someone would've called them on it if they just blatantly made things up - therefore the Gospels can be assumed mostly true. The Gospels say many people saw Jesus die on the Cross and then saw him alive later, and that natural explanations (Jesus survived the crucifixion, everyone was hallucinating, it was Jesus' twin brother - yes, they actually addressed that) are all unconvincing; therefore Jesus really was resurrected. According to the Gospels, this was seen by many witnesses, including luminaries like St. Peter, and none of them later came forward to say "No, we didn't see this at all, shut up". Further, many of them later died an extremely predictable martyr's death, proving that they believed in Christ's resurrection enough to sacrifice their lives for him, something they wouldn't have done if it were all made up (they point out that although some people, like kamikaze pilots, have sacrificed their lives to false philosophies, it is far more unlikely that the Apostles would sacrifice their lives to a false empirical fact, namely that they had seen Jesus rise from the dead).

Multiplying the low probabilities of everyone involved simultaneously having some kind of fit of insanity leading them to sincerely believe Jesus had risen from the dead gives 1 : 10^39 against, and since this is a very small number obviously the argument must be correct.

This argument doesn't quite take the truth of the Gospels as a premise, but it comes close. Although there are some atheist accounts that allow for the truth of the Gospels as written while still casting doubt on Christ's divinity, that's not where the smart money lies - most atheists would deny to one degree or another the validity of the Gospels themselves. Either the entire thing was made up (a theory which the McGrews reject, and I think rightly) or a historical Jesus had various miracles falsely attributed to him by overzealous believers. This leaves the McGrews' objection that the existence of a wider Christian community, many of whom had been personally involved in the events described, would have limited the Gospel writers' ability to make things up even if they had been so inclined.

So instead of basing his argument on the likelihood of people hallucinating resurrected Jesus, the McGrews should have investigated the probability that the Gospel writers would make up miracles and the probability that they would be caught; something like

P(resurrection) ~= P(gospels true) ~= 1 - [P(people make stuff up about Jesus) * P(they don't get called on it)]

So what is the probability that, given some historical tradition of Jesus, it will get embellished with made-up miracles and people will write gospels about it? Approximately 1: both Christians and atheists agree that the vast majority of the few dozen extant Gospels are false, including the infancy gospels, the Gospel of Judas, the Gospel of Peter, et cetera. All of these tend to take the earlier Gospels and stories and then add a bunch of implausible miracles to them. So we know that the temptation to write false Gospels laden with miracles was there. Apologists say that the four canonical Gospels are earlier and more official than the apocryphal Gospels, and I agree, but given the existence of a known tendency for people to make up books, and a set of books that sound made-up, the difference seems more one of degree than of kind.

That leaves the question of whether anyone would notice. The dates of all the Gospels are uncertain, but around 70 - 80 AD for the synoptics seems like a fair guess. The average life expectancy in classical Judaea for those who survived childhood was 40 to 50. That means Jesus' generation would be long gone by the time the first Gospel came out, and even people who were teenagers at the time of Jesus' crucifixion would be dying off. Christian tradition lists all the Apostles except John as dead by 75 AD.

There's also the more general question of argument from silence. Let's say someone did have evidence against something in the Gospels. Most Judeans at the time wouldn't have been literate, especially not in the Greek in which the Gospels were written. Many who were, might not have had the time or interest to pen responses to what seemed a minor cult at the time. If any did, those responses might not have spread in an age when every work had to be laboriously copied by hand. And if by some miracle a refutation did become popular, there's no reason to think we would know about it since many of the popular works of the age have been lost completely.

Matthew mentions that on the day of Jesus' crucifixion, graves opened and the dead walked the earth throughout the city of Jerusalem for several hours. No one else (including the other evangelists!) mentioned the dead walking the earth, either to confirm or refute it, so clearly the 1st century AD Judean skeptical community wasn't exactly on top of its game. That alone casts suspicion on the whole "if this was false, someone would've said so" argument.

All of this makes the Gospel argument relatively uninteresting to me. But it hints at a different problem which is interesting. Twenty years after the death of Christ, we have Paul writing letters to flourishing churches all across the eastern Mediterranean, all of whom seem to have at least a vague tradition of Christ being resurrected and appearing to people. That means Christianity spread really, really fast, presumably by people who were pretty sure they had met the resurrected Christ. At my current, limited level of Biblical scholarship I consider myself still confused on this point and yet to see a satisfactory explanation (people rising from the dead doesn't count as 'satisfactory').