Ann: Mesoblast Update on COVID-19 ARDS Trial, page-491

If you look at MSB's stem cell story its evolved from injections of stem cells that were supposed to "repair and replace" injured tissue like knee cartilage or herniated interverterbral discs or heart tissue. When trials didn't show any real evidence of this there were numerous post hoc sub group analysis on softer targets such as "reduced pain" or surrogate "inflammatory indicators" or "gastric bleeding".

Lots of anecdotes and enthusiasm, but short on clearly successful RCT data and a coherent and logical explanation as to how it works. Any scientific proof requires a mechanism of action so it can be further investigated as part of the proof. There's no bias against it from the FDA. They are simply asking for scientific evidence to back up the claims from the company which have so enthralled investors.