I will keep an eye on them also. I think they are using service. Because some of the accounts are from 2014 but they are only posting on ann thread. So, those accounts must be the accounts of that service seller.
But you can also be right they might use the airdrop system to bump the thread. let's see the next round of airdrop.
There is one more thing that makes me lean towards conclusion that this spam was made by airdrop: if you check that archived version of their ANN (since all those spam posts have been deleted now) you will see that most comments were very short, like "great project", "good project" "thanks" and similar spam. While posts made by bumping services are not much better, they are still a bit longer, and usually made in form of question, so they at least write one whole sentence, and not word or two like it is usually the case with airdrop spam.
Anyway, let's see what happens when they start 2nd airdrop round.