Heparin will just attempt to break up the clot and heparin...

Heparin will just attempt to break up the clot and heparin agents do agents have some anti-inflammatory properties but not in the same league as MSCs. Like taking a knife to a gun fight.

Again, would be better to prevent the on-going damage which continues to spread through more organs, with more micro-emboli developing, which themselves are pro-inflammatory, by using MSCs. Makes sense to me.

Patients who are bed-bound are usually given DVT prophylaxis in any case, and can go from there to full anti-coagulation, which is considered on a case by case basis, but some Covid cases may be at risk of bleeding (perhaps counter-intuitively) complicating their management. So again, stop the clotting problem at its source (inflammation) with MSCs and you avoid potentially life threatening clotting vs treatment-related bleeding.

This dilemma occurs in treating cancer and in treating pre-eclampsia (an inflammatory condition associated with pregnancy), so we are used to the balancing act. If it can be avoided, eg now with MSCs in this condition (Covid-19/ARDS) that would be a game-changer, and most welcome. IMO.