Twitter absolutely sells books. In particular, it sells:

--ebooks (instant gratification)
--reasonably priced books (low cost experimenting)
--books to voracious readers who want lots of recs
--books to people looking for something specific/hard to find in mainstream pub https://twitter.com/lydia_petze/status/1200357232262565888

The authors saying "Oh, Twitter is nothing, you need your publisher to get the book stocked in independent bookshops" or whatever are speaking about a section of the book world that is very much not all.

Twitter has strong reach for me because I've tapped into a large network that feeds out to voracious one-clickers. (That's you. I love you. It's also me. /looks at 27 books bought this week/)

If a SM platform works for you, it works. Please don't panic if you can't afford BookBubs and ads, if you aren't tradpubbed and getting your hardback hand-delivered to Waterstones by liveried flunkies. You can do just fine.