This is a common reaction to a pervasive phenomenon

Reading is one way people encounter ideas and learn, and so some people who read have a broad worldview and have absorbed the insights from books

People start to notice this, or perhaps those people attribute some other success to reading books. Enough association of this kind makes reading books high-status

Many people want to short-circuit this process to get status, so they optimize for reading a lot of books, or sometimes fake reading books

People who fake reading books for status are shown ridiculous when exposed

People who are allergic to and resent status nonsense conclude that bookshelves mean someone's a poser

This is obviously ridiculous. Many people have bookshelves because they simply need to store some books

The error committed here is, to me, a common one, overfocusing on the most salient case. I agree that it's icky when people do shit like fill shelves with books they've never and will never read as a status symbol. But are we really to assume that this is the most common reason to have a bookshelf? Or is it just the reason to which you have the strongest reaction, and thus stands out the most to you as a hypothesis?

With some additional information about a person, one might be able to conclude this given the bookshelf, but it hardly seems like enough evidence to come to this conclusion by itself

That's a poor way to look at the world. Also have no interest to change your mind.

"It is foolish to think that you have to read all the books you buy, as it is foolish to criticize those who buy more books than they will ever be able to read. It would be like saying that you should use all the cutlery or glasses or screwdrivers or drill bits you bought before buying new ones.

...we understand that it is good to have many at home rather than a few: when you want to feel better, then you go to the ‘medicine closet’ and choose a book. Not a random one, but the right book for that moment. That’s why you should always have a nutrition choice!"

-- Umberto Eco (https://fs.blog/the-antilibrary/)

Do you mean bookshelves as in the place where you put books before, during, and after reading them, or bookshelves as in the online habit of sharing what you've read?

I agree that the latter is self-congratulatory but I don't see why you'd object to the former.