Drilling the hole is just a little insurance to give the glue a channel to wick deeper into. The slot in a slotted hinge does the same thing.

I often do not even drill the hole if the fit seems right - it's more important on a tight-fitting slot. But then, I've been doing this for a long time and I can judge when it's necessary and when it's not.

So the bottom line is - If you've got a slotted hinge, the hole is not necessary.