I believe options that force a player and a DM to actually think about who they are as a character and what they are doing within the world is a good thing. Otherwise... you might as well just remove all flavor text from abilities and instead say "You can do X in the board game" if the flavor text has little to no meaning.
I'm perfectly with player and DM coming up with meaning instead of being straight-jacketted into a single WotC-approved flavor text. Easy example, was in an Ancient Greek inspired 2014!5e campaign and I ran a satyr hunter. Mechanically it was a rogue. Great perception, could sneak up on prey, take a shot from hiding that could drop them (sneak attack), etc. But zero flavor about stealing or assassination. Worked great, and fit the concept better than the ranger class with it's built-in casting would have. But if I war required to play to roguely flavor I wouldn't have been able to do that.
There are an incredible number of concepts for characters, and honestly plenty of ones from stories or movies that don't align the best with D&D mechanically. If we then limited D&D mechanics only to an "allowed flavor", we'd be making a whole lot more of them unable to be realized in the game.
All of that said, I actually support what you said specifically for this feat. Everyone has a class, they need to be broad. Even a class like paladin has been loosened from "chivalric lawful stupid" as an architype from early D&D to allow a much broader spectrum. But there are a lot of feats, and having some options chosen from a large list to have meaning and implications isn't nearly the limitation of everything being fettered to flavor text.
What's the point of letting a player "become a Lich" within the story of the campaign world if there's nothing for that player to actually think about in terms of actually being a lich in that campaign world? If "becoming a Lich" is merely just a couple game mechanics you take at certain levels... it's no different than any other game mechanic you could take instead. You are no longer becoming a Lich... you just have a couple additional game mechanics to use during the tactical miniatures board game.
Like I said I agree with this particular feat having flavor. But as a more general issue this seems to be mistakenly conflating "not using WotC's flavor text" with "has no flavor text" which is just incorrect. Players and DMs can and do flavor mechanics all the time. It's fundamentally incorrect to think that if the rule book does not impose a specific flavor that there is no flavor and apply the rhetoric of calling it a "tactical miniatures board game". Heck, we even have official rulings like the one in Tasha's to feel free to reskin spells to fit your character's flavor. So officially, publishing in the rules, there's an example that ignoring WotC flavor is fine, and that player/DM flavor absolutely exists.