The upshot of this is that LLMs are quite good at the stuff that he thinks only humans will be able to do. What they aren't so good at (yet) is really rigorous reasoning, exactly the opposite of what 20th century people assumed.
Alfred Bester's "The stars my destination" stands out as a shining counterpoint in this era. You don't get much character development like that in other works until the sixties imo.
Yeah, I'd say characterisation is a weakness of his. I've read Stranger in a Strange Land, The Moon is a Harsh Mistress, Starship Troopers, and Double Star. Heinlein does explore characters more than, say, Clark, but he doesn't go much for internal change or emotional growth. His male characters typically fall into one of two cartoonish camps: either supremely confident, talented, intelligent and independent (e.g. Jubal, Bernardo, Mannie, Bonforte...) or vaguely pathetic and stupid (e.g. moon men). His female characters are submissive, clumsily sexualised objects who contribute very little to the plot. There are a few partial exceptions - e.g. Lorenzo in Double Star and female pilots in Starship Troopers - but the general atmosphere is one of teenage boy wish fulfilment.
Generally, I also agree that Heinlein's characters are one dimensional and could benefit from greater character growth, though that was a bit of a hallmark of Golden Age sci-fi.
There is much worthy of critique in Heinlein, especially in his depiction of women. I've spent about a quarter century off and on both reading and formulating such critiques, much more recently than I've spent meaningful time with his fiction. I've also read what he had to say for himself before he died, and what Mrs. Heinlein - she kept the name - said about him after. If we want to talk about, for example, how the themes of maternal incest and specifically feminine embodiment of weakly superhuman AGI in his later work reflect a degree of senescence and the wish for a supercompetent maternal figure to whom to surrender the burden of responsibility, or if we want to talk about how Heinlein seems to spend an enormous amount of time just generally exploring stuff from female characters' perspectives that an honest modern inquiry would recognize as fumbling badly but earnestly in the direction of something like a contemporary understanding of gender, then we could talk about that.
No one wants to, though. You can't use anything like that as a stick to beat people with, so it never gets a look in, and those as here who care nothing for anything of the subject save if it looks serviceable as a weapon claim to be the only ones in the talk who are honest. They don't know the man's work well enough to talk about the years he spent selling stories that absolutely revolve around character development, which exist solely to exemplify it! Of course these are universally dismissed as his 'juveniles' - a few letters shy of 'juvenilia' - because science fiction superfans are all children and so are science fiction superhaters, neither of whom knows how to respond in any way better than a tantrum on the rare occasion of being told bluntly it's well past time they grew up.
But they're the honest ones. Why not? So it goes. It's a conversation I know better than to try to have, especially on Hacker News; if I don't care for how it's proceeding, I've no one but myself to blame.