An advert for rationalism, yes, but that is a way bigger topic than the things you mentioned.

To me that seemed to be the payload. The rest is just window dressing.

It's been years since I read it. How does it hype up cryogenic preservation? I don't recall it being a major theme and it doesn't seem to fit into the HP context.

You are right that it doesn't fit in. You are wrong that it isn't a major theme. Probably the last third of the book is about cryogenically preserving someone while looking for a way to revive them.

Ok. It's possible I read it before that part of the book was released, but it's equally possible I forgot about the plot.

See comment below regarding dementors. When I read those passages it suddenly clicked for me what the whole thing was about. Easy to miss though, it's not as if they are advertising it.

This is a story where our hero is the heir to an ancient family legacy of pursuing immortality, where the most vile creatures in the universe are afraid of him because he thinks death sucks, and where the climax of the story is healing a cryogenically frozen person and bringing immortality to the masses.

It's not "easy to miss". The author beats you 'bout the head with his ideology. It's heavy-handed even for something explicitly didactic.

No, the thing it was most supposed to be an advert for is AI alignment (at the time, particularly MIRI).

"A True Patronus can destroy them, because it is fueled by the thought that Death does exist and that we must face it, but that we will one day defeat it."

https://hpmor.fandom.com/wiki/Dementor