It is a “transaction” type of thing.
Similar to when a member makes a poorly formatted post, the tries to edit it.
If at the same time a Mod fixes it before the member finishes the Mod fix will be replaced with the member fix when they Save.
I think most times the chances of more than one working on a post / topic at the same time are rare. Though the problem has come up before, wiki edits in particular.
I know databases can lock transactions, but I don’t know how it could be dealt with here.
I have seen it suggested to prevent further topic replies if another is in the process of replying, which I think is a poor idea.
But it might be nice to have on a per post level.