hahaha software engineer here - maybe I'm just stupid, but no it's not obvious to me. What I'm going to take out of that is that ESXi doesn't do a copy/delete in the form of a move, I'm going to assume it's doing a byte by byte move (which is why I'm assuming you're saying it's obvious).
More just my talking out loud than a direct response:
If that's the case, as a guy who has to write frontend UI code, I have to wonder, if the task can't be canceled why is it an option then? Might as well just hide the button if it isn't going to do anything. Are all of the users of ESXi just supposed to have memorized what operations can and can't be canceled and ignore the cancel button appropriately? I wouldn't write into my frontend a button that the typical user is going to assume does something, but then on the backend secretly have it do nothing! 
To your point - ya, I have a lot of tasks that I'll hit cancel and then after a minute or two it will eventually say canceled. Which is actually what makes this particular situation even more annoying - am I just being impatient? Is it doing nothing? How do I tell?