I'm 100% sure that the Tank mount achievement includes all lvl50 content, just that they are adding Extremes/Coil to that achievement as well, otherwise they would be excluding over half the community who prefer to run lvl50 dungeons over raids. They aren't rewarding just raiding tanks but all 50 tanks. This would make sense as they want to encourage more tanks to do dungeons and raiding at lvl50, not one or the other.

I just got my alt to 50 warrior so I will look forward to completing this achievement one day. Same goes for my main who is healer but might take up moonlighting as a tank in dungeons for the achievement as well!