They used to make the LFG posts directly. Bungie made it so you need to own one of the newer DLCs to create posts (anyone could join, regardless of DLC) - this cut the spam considerably since the bot accounts aren’t going to pay for any DLC to just get banned and lose money.
Now they scour the accounts that make/join legit posts and exploit Bungie’s accounts through DM spam. If the bot gets banned for advertising through DM, they just make another throw away account.
It’s only a matter of time before Bungie adds SMS verification to DM, or more insane requiring DLC ownership for that too.