<...>
It might just be that they post it, and then don't monitor it much afterward, which definitely wouldn't be the way I go about it, but I guess some users do that.
Yes. It seems they were hired by the project owner just to create and publish an ANN topic. Much like the translators in the local sections do.
<...> this means that the service provider paid for these bumping services... for nothing! I really don't understand the logic behind this. It's like... OP was not even interested in selling his product anymore... like he simply paid to harm the forum through spam!
There might be a simpler explanation. Someone may have set up a bumping bot and forgotten about it.

While I don't know too much about it, I suspect something like that is possible.