So a mining pool isn't allowed to go into a coin's thread and mention that they've added that coin?
No.
If you'd like to send a personal message to the OP then you can do, maybe if they deem it appropriate they may add it to their website that you can mine their coin at x mining pool, and include a link to their website pool page on their thread. Other than that its not allowed, and a lot of the owners of these announcements actually report these sorts of posts.
This question has already been answered directly on the unofficial guidelines thread by mprep:
22. Advertising (this includes mining pools, gambling services, exchanges, shops, etc.) in others threads' is no longer allowed, including, but not limited to, in altcoin announcement threads.
This is a more up to date version than the 2014 post that you are referring too.