Ann: Phase 2/3 Trial in COVID-19 ARDS Begins Enrollment, page-48

Mesoblast Limited .... has commenced enrollment.
They now need to go through the enrollment process..get consent etc. Then they can dose.

More than 20 medical centers across the United States will participate in the trial which is expected to complete enrollment within three to four months.
Enrollment takes time. It doesn't happen overnight. It may happen quickly, may take time if families need convincing. This, I think, was the advantage of news coverage of the 12 test cases.
Also, 300/20 is 15 cases each. Wouldn't think it would take long in New York!

Ventilator-dependent patients in intensive care units with moderate to severe COVID-19 ARDS randomized (1:1) to receive either two intravenous infusions of remestemcel-L within five days or placebo on top of maximal care.
The controls are no worse off by taking part in the trial.

The primary endpoint is all-cause mortality within 30 days of randomization, with the key secondary endpoint being the number of days off mechanical ventilator support.
It will take 30 days for the first case to complete, longer for those enrolled later hence the stretch out in time.

This is my reading of the communication.