You seem to be pointing out that formal education isn't needed to learn something. That's obviously true. The person you're responding to seems to be saying that being a librarian is a job that requires a nontrivial amount of learning. That learning is on the level of something like a masters degree. This is also true

That's how it ought to be done in academic libraries. It does take a lot of librarians. You have a librarian for each subject area, which can be pretty granular depending on the size/budget of the library. My academic library has a business librarian who focuses on acquiring and weeding the business section. She has an undergrad degree in something businessy and an MSLis.

Now, my academic library has been subject to funding cuts and so we don't do everything we ought to do, especially in areas that aren't major majors at our school, which is frustrating AF. We rely more on professors suggesting acquisitions.

Public libraries have a broader remit, so often can go with purchasing what's requested by their patrons and what's on lists of good new books. They still have to know their subject matter to weed properly, especially in the nonfiction areas. Fiction tends to be weeded based on condition and circulation stats, especially in public libraries. (weeding is getting rid of books that no longer suit the collection. Easy examples: children's book that was dropped in the mud. children's book that still lists Pluto as a planet).

Oh! As a librarian, you can also get pre-release versions of books to evaluate for your library. It's a nice perk to read things months before it comes out. (you have to request them from the publisher, but publishers like sending these to librarians because librarians buy lots of books and talk to other people that buy lots of books)