Should citations of designer intent alone be sufficient for a valid answer to game-recommendation questions? That is, in the absence of any description of actual play experience?
No, game-rec wants what actually works, not what is supposed to work
The game-rec rules rely on one simple premise: you, or someone you can point to, has actually tried what you recommend, and it actually worked.
A statement of designer intent is a statement that something should work, but that does not mean it actually does work. And "actually does work" is the basic requirement of a game-rec answer.
Designers do, of course, play their own games. Designer statements describing their own play of their games are fine, great even. But the statement alone that they meant for a game to be good for some purpose is insufficient.
Game-rec guidelines allow citing other people's experience using a game for the exact use described in a question. Designers are people too.
In practice it's very rare for there to exist clear, citable statements from a game's designer that it functions usefully for X, where X is an exact match for something in a game-rec question, so it's not really an issue. If someone did cite an exact match, it would make for a very good answer.