It's an odd sort of "review" that mentions the notional subject mainly in passing, but no matter: his remarks on Heinlein are right to the point.
In particular, I like his emphasis on Heinlein's gift for effortless simplicity. Heinlein always reminds me - now don't laugh - of Joseph Haydn. Haydn was one of the few true greats. He invented many of the forms which would shape and dominate music for a century or more (including both the symphony and the string quartet), yet on face value his work seems remarkably simple. It's pleasant, very easy to listen to, and entertaining as well, but rarely takes your breath away with the spectacular showmanship of Mozart, Beethoven, or Bach. It is only on repeated listening that you gradually realise how much depth there is in it, and start to feel a bit foolish when you remember that at first you though it "simple". I don't think one can learn that sort of "simplicity". It's a gift.
Take Glory Road as an example. It is just a straightforward fantasy adventure simply told, and because the telling itself is so unobtrusive, the tale itself lives in a way that more "advanced" writing cannot emulate. Or consider Double Star where we see the same thing. Even when Heinlein decided to do something obviously different - I'm thinking of The Moon is a Harsh Mistress - his gift for economy remained effective.
Heinlein's young adult novels are unique. Heinliens book length fiction peaked between 1950 and 1960. (His best short stories were written between 1940 and 1950.) His best 'adult ' novels are Door Into Summer and Double Star. You can skip Rocket Ship Galileo, he seemed to be feeling his way there, but Space Cadet, Red Planet, Between Planets, Farmer in the Sky, Rolling Stones and especially Star Man Jones are real gems. These are not blood and thunder stories but are what one might call 'domesticated science fiction'. Heinlein always had the knack for 'lived in futures' , hard science was used as an integral part of the stories and one felt the verisimilitude. Heinlein followed Star Man Jones with The Star Beast ( I thought was a bit 'juvie'), and then Tunnel In The Sky, I didn't think the plot of this one was so smooth (by the by this novel has a 'star gate' as does Star Man Jones (which seems to have a traversable wormhole!) Time for the Stars had a slow paced story. Citizen of the Galaxy shades towards the adult its good but over elaborated. Have Space Suit Will Travel returns to old form this is a cracking good story. Star Ship Troopers was written as a young adult but Heinlein's 'juvie' publisher would not touch it , so it became an adult novel , just as well , it is a good story regardless its somewhat hard line philosophy.