Ann: Remestemcel-L for COVID-19 MIS-C published in Pediatrics, page-152

Well that needs to be qualified.
Statistics involves levels of confidence and significance so we have degrees of certainty that an effect is "real" and does exist under the specified conditions, rather than just the result of "chance". And we have one or two tailed tests as well...

But scientific proof requires a more detailed investigation of the hypothesis with a mechanism of action and a pathway that can be blocked or interrupted or otherwise tested by experiment. Sometimes the statistics will tell us that something "should exist" and then we need to build a better microscope or telescope to see it.

But statistics usually can't resolve absolutes. Often things may work to some degree in some patients. And other times they don't work at all. And useful medications work reliably in the majority of patients, especially those with a known and scientifically proven mechanism of action.