Yes a subchorionic hematoma is a clot or tear in the placenta. But you dont want to be breaking up the clot with a blood thinner is my impression. For it to heal the clot would eventually reabsorb or be bled out(on its own) and then eventually the placenta will heal, but if you are taking a blood thinner I imagine it will take even longer to heal because you will bleed easier. Also, it may not be a clot causing the bleed site, but a seperation in the placenta which is what I had I was told. I never bled any clots, just blood, and for it to heal the bleed site on the pacenta will need to clot up a little(picture a wound on your skin, it wont heal as long as it bleeding, once the bleeding stops it can in a sense clot up and eventually heal).
Its weird to me that your Dr wants you to still be on it, if you only took it to aid in conception and have no other reasons. A hematoma shouldnt be the reason to be kept on it, to me it makes no sense. Did he say why he wants you to still be on it? I am part of a forum of women who have or had a hematoma and not once were any of told to take aspirin.
EDIT: Also a subchorionic clot is NOT the same as the clot you get that you do need to take aspirin for. They are 2 totally different things.