MMR data over the course of the last X months (not sure if it was 6), not just at a point in time. MMR moves with every match, so they’ve been watching the matches with role queue in mind, so when you played a tank in comp any time over the last X months they used that performance to do an MMR adjustment on your new tank MMR. The same with each of the other two roles. Every time you’ve played a hero in each role in comp, they’ve allowed that to adjust your role specific MMR, as well as your active overall MMR.
The only thing that doesn’t count is the beta season from what I can tell.
Jeff talks about it in the video below.
They used the best MMR value they could going into placements, then based on that, plus the performance you had in your placement matches to determine where you belong.