I also find it unlikely Harry hadn't heard of cryonics. I mean, I'm born in 1981 almost like Harry, and when I was Harry's age, I definitely had heard of it, like I had heard of robots, or long-distance space travel, or life expansion, or robotic prosthesis, as a scifi device. I had seen 2001 and Star Wars. And I did think at that time "oh, it's something cool we'll have in the future". Sure, I didn't know it was something actually existing in the real world. But while I wasn't a dumb kid, I wasn't the supergenius Harry is, and I didn't even think defeating death was something possible.

So, yes, I would say it's unlikely for Harry not knowing about cryonics. But... not "1 chance in 1000" unlikely, more like "1 chance in 10" unlikely. And if you have 10 "one chance in ten unlikely" things, well, statistically one will actually happen. And it's fully within the author's "rights" to say "well, among those ten things there is only one chance in 10 Harry doesn't know about, it's cryonics he doesn't know about".

So while it's unlikely that Harry didn't hear about cryonics, it's not unlikely enough to really sound artificial. Harry knows a lot, not all, and cryonics is where he fails, ok, fine.