To add to what I said above, I think someone might have asked this question before a very long time ago and if I remember correctly theymos even commented on it if someone wants to do some digging to see what he said.
Yobit and Cryptotalk had their signature campaign to shill Cryptotalk forum in
2019. Because initially it was a spam campaign, Bitcointalk community reacted to it and yahoo took over the campaign to clean spammers from participant list.
theymos even banned the Yobit account here because of abuse and some spammers from that campaign but eventually, he allowed Cryptotalk forum to advertise here via a signature campaign under yahoo62278.
I don't think that was it. I think someone else directly asked whether it was OK or allowed to advertise a competing forum and thyemos gave his opinion.