LR, if the concentration is expressed in Mols, as I think it is in this case (10 uM keeps coming in mind) then it would mean that both B and R have the same number of molecules per unit of volume AND biologists could say:
".. B is more effective in the inhibition of the coronavirus relative to an equal amount of R"
That's the beauty of using Mols.
Now, If Leo & Co are using something like weight/volume then the situation is a tad different. In this case you have equal weights per same volume. That is equality in, say, on scale used to calculate shipping cost, but not in biological sense.
I always make sense, don't I. Clear as day etc