That is a dangerous state for a rationalist to be in. so would you please be a dear and have a look at that book to figure out how precisely it junk-scienced me and the rest of its readers? It would really help me out and I'd really be grateful for that.

I'm afraid I haven't read that exact popularization, but if it's drawing on Ericsson's research as it sounds like, the explanation is easy enough: Ericsson's points are valid largely because the studies are correlational, do not control for underlying factors or Matthew effects, and suffer from heavy range restriction in he's already looking at people who are selected or self-selected to be elites.

(ie. suppose someone studied MIT physicists with a mean IQ of 150 and discovered that in this group of physicists, Conscientiousness predicted better than IQ which would go on to win Nobels. This is a possible result, and what this has actually demonstrated is "you have to be incredibly brainy to be a MIT physicist in the first place, but once you've gotten that, then other things are also important; which is another way of saying that if we look at the general population, like all the people from IQ 60 to 150, IQ is the overwhelming most important trait" but this is easily popularized to "IQ doesn't matter!")

Also, I have a vested interest in believing in the "only deliberate practice matters" thesis. I used to feel having a high IQ obliged me to get excellent results with little effort, and every time that didn't happen I felt disappointed in myself and that I didn't live up to some weird standard. Nowadays I only believe in working as hard as possible for as long as possible, and it serves me much better.

As a practical matter, it's probably a good idea to believe hard work matters and deliberate practice matters. If you could somehow improve your intelligence, then it might be important to hold correct beliefs about IQ being far more important than practice; but unfortunately, IQ is pretty much fixed and all that's left is to make the best use possible. Given people matched on IQ and other traits, and Conscientiousness will be pretty important.

However, in other contexts, it's very important to hold the correct beliefs about the relative value of intelligence and 'just work harder' - if we were discussing iodization or immigration or whether someone should go into debt for college, for example. Many population-level questions will rest far more on intelligence than other traits.

Understanding which context we're in can be a hard balancing act, and especially difficult when reading papers making statistical claims (did they control for IQ? Should they control? Or for education? Or for range restriction? Or for reliability of their metrics?); I try to be consistent and clear in any discussions of Conscientiousness or IQ which context we're in and which we should value or ignore, but I don't think I always succeed.