The charging circuit will maintain a "safe" temperature, sure, but using any battery monitor app you like, you can clearly see for yourself that when wireless charging, the phone will sit at a higher temperature during the process, than when wired charging. And every fraction of a degree matters. Not when you use the feature once, but if you do it every day, always charging at a higher temperature, it WILL shorten the lifespan of the battery. The same way every charge does, but to a slightly higher degree for every fraction of a degree in higher temperature.

The battery does not suffer as much damage from heat when discharging, or when just sitting there, not that that is good for it, either. But every extra bit of thermal strain WHILE charging, causes more damage than if it were running cooler. That's simply a fact of lithium ion batteries.