Not seeing the issue. By virtue of it working with Qi at all, we can assume it meets the “normal” minimum coil size.
The issue with using a Qi charger that tiny is that you're balancing your phone on something quite small. And if you've ever used one, you are aware that correct alignment is all-important.
Not hard to imagine that a tiny puck + no magnets = a very fiddly experience getting it lined up. And that's not even touching on the fact that if your pre-12 iPhone better not get jostled at all since it's got no magnets to keep it in place.
Although, at that point I guess you could just lie your phone face-down and put the puck on top of it. Fun!