I also think it is partially retroactive now from what I've seen of who has this. Since the Summer 2012 DLC came out, it has actually been logistically impossible to play 2500 hands of Heat even if you played every session so far.
I played a ton of Heat when the game first came out in Spring 2011, so most of my Heat hands were from that period. Several people I've found on the leaderboard who have this do NOT have any of the Spring 2011 achievements. So I am guessing that what Wimp Lo on TA suggested is true in that only the more recent seasons count. I am up to 1250 hands of Summer 2012 Heat and have 15,000 total hands played, so looks like most of my early Heat didn't count.