You were right, they are obviously using one of those bumping service. Bunch of the accounts that they are using  were inactive since October 2019, and then suddenly woke up and started spamming there. Their ANN has been added to the list.


There is one more new addition, making it 53 at the moment:
[ANN] Minereum - 🔥 MNE STAKING IS LIVE! 1000% REWARDS + 60% REFERRER BONUS 🔥
Minereum ANN was created long time ago, back in 2018, but recently they decided to increase the visibility of their project so they hired bumping group. Accounts that wre working on thier thread are alos active in DSF and SImba, two ANN threads that have been added recently to the list.

Same accounts possibly started working on Tranquility Audit Security ANN, but I will wait a bit to see to see what's going on before adding them as well.