The observation is correct. Remsedivir is a prodrug, the active compound looks like adenosine monophosphate. But that molecule bears too much charge to be able to cross cell membranes. Consequently medicinal chemists put protecting groups on the offending atom, to help the compound get out of the bloodstream and into cells. Once inside, those are cleaved off through nonspecific esterases and the molecule can't get back out.

It's all part of ADMET (absorption, distribution, metabolism, excretion and toxicity) optimization. Pharmacokinetics is an important subject, and that's why you can use the "XY shoved activity in vitro" papers only as starting points.