This one is a tricky one. It does fit the definition of an exploit as it does in fact exploit a hardware lock state (that’s how NumLock functions). However, to fix this would almost certainly require reworking the entire spell/ability system with regard to the OnKey, KeyUp, and KeyDown states, which would very likely break a lot of the legitimate gameplay players enjoy now. It’s also pretty much impossible to detect, especially when you have BIOS/UEFI settings that can set NumLock to on/off, making for more variables in that equation.
So the most realistic educated guess for this is that leaving this be is the lesser of two evils. Yes, it provides an exploitative behaviour, but fixing it could well break everything else, and would also be a ton of work for little gain. This one falls squarely under the “not damaging enough to break stuff over it” category. It isn’t reading D3’s heap stack or any such thing. It’s literally just a hardware lock state.
It should be noted that this cannot be exploited on the OS X side, as that OS doesn’t use NumLock in the same way Windows does.