This was based on comments about his work, that I studied over a decade ago. Not a one off assumption based on an idea presented in his work.

Of course not, that was in his libertarian polyamory phase. That is not a joke, that is part of why he wrote it.

I think the biggest thing people struggle with on Heinlein is that he was prolific and enduring enough that, like any reasonable human being, his opinions changed over time.

He literally wrote Starship Troopers because he was upset that America was suspending nuclear weapons testing. He wrote Stranger in a Strange Land after seeing that the US government might not be worth trusting 100% and realized that he really liked fucking other women besides his wife and figured what was good for the goose was good for the gander. He also made the Martians both socialist coded and demonstrably superior ethically to the hypercapitalist Earth government.

Neither is necessarily the entirety of his worldview as expressed by the characters but they didn't come from nowhere either, and he doesn't portray any of the systems as flawless.

For what it's worth, he ran for office in LA as a Democrat during the Great Depression and was generally pretty progressive. The reveal in the Starship Troopers that "Johnny Rico" was a Filipino officially named Juan Rico was a deliberately anti-racist message in the Civil Rights Era.