imho the reason why Phaser docs are so bad is an excuse to sell... Books about Phaser. Maybe I'm wrong, but why they exists if community repeated many times that docs should be improved?
Hanlon's Razor. It seems to me that if there's limited resources, then if they've decided that they're not using "documentation as marketing" (which for free frameworks doesn't seem to be the best strategy to be honest), then directing time to developing paid for learning materials over and above using that time for free docs is a valid strategy.
I can't imagine any project deliberately having crappy docs in order to sell resources, as you'd get almost no buy in to begin with.
everyone is entitled to an opinion, even if that opinion is quite clearly wrong. Phaser is a tiny team of very dedicated individuals and they have successfully produced one of the best JS/TS game frameworks. If you want better documentation, why not produce some? Want to help the team grow so they can afford to employ someone to write docs? Buy the books, pledge to the Patreon. Basically: help in some way. That's much more productive than suggesting they did all this work simply to sell some books.
An opinion can't be wrong, a statement maybe, but a statement "this is what I believe: <opinion>" is in any case not up to debate. Because An opinion has to be supported by arguments and those can be wrong. But of course, you are entitled to your opinion.
I can't imagine any project deliberately having crappy docs in order to sell resources, as you'd get almost no buy in to begin with.