Why does a dent to a tweeter not affect sound quality?

Whether the damage presents an audible difference or not, sellers of these damaged listings should expect to bear the depreciation, just as if they had any other damage (dinged corners, scratches, cat torn grills, surround dry rot, etc.) If it didn't matter, dealers would have no problem with potential customers poking in their tweeters for fun.

you can apply gentle suction (such as sucking through a cardboard toilet paper thingie) and pop it back out.  Whether it remains popped out is another matter.

That works with soft domes, but not metal.

I usually push them in on day 1 so I don’t have to worry about it later. I have to suck out the inverted domes on focals with a vacuum.