My guess would be it's to build up reputation of the sending accounts, by being "connected" to trusted, aged, accounts? However, the combination of garbled text + number @ large provider on its own should be pretty suspicious by now. Another idea is some kind of affiliate marketing / pumping attack, but I don't see ads.

Yeah, I think you might be right. I was also thinking that it could be a weird setup of Moltbook-style LLM training?