You must have missed the "before it becomes common" part of the post you've replied to. No one is suggesting it's being used a lot. The ban is to stop it before it does.
Apollo Constellation Engine designed their propulsion systems for mega constellations to use mercury, which is what got PEER involved in this. Popular Mechanics also reported on their plans. Even if Apollo changed their minds since, that's a pretty significant 'near miss'.
Becoming common still needs for there to be some usage, even before it becomes common.