True but lots of people are not great at drilling holes, can make it worse. Drilling at the top of a slit, the lack of any metal on one side of the hole makes the drill want to wander to that side. Do-able of course but some drilling knowledge and experience is required. One option is drilling just above the slit so the drill doesn't have as much desire to wander, then connect the slit up to the hole, but that has its challenges too, and it's reducing the amount of un-slit quill, lengthening the slit. Drill the smallest hole you can (that still removes the sharp corners) because any metal removed is likely reducing the strength of the quill.

I think for most people, a needle file is safer, though slower. Just file a somewhat-rounded shape at the top, minimum amount of metal removed until the two 90° corners are gone. The hole doesn't need to be round, just "rounded".

Many people don't own any needle files but they're cheap and easy to find, so go grab one. Typically they come in sets with various shapes including triangle, flat, thin wedge etc, but the round one will get used 5x as much as any other, so ideally get a set plus a couple other small round ones in several sizes and coarse/fine. "Import" quality (code for China) is good enough for occasional use on soft metal like alu h'bar stems. Seek out better quality (like Grobet) if you dislike buying badly made tools.