You really don't want to hook up a 48V POE injector ( 802.11at or af but especially 48V passive) to this gadget. It will let the smoke out of a 24V passive POE device.
This. It probably uses what is called a buck converter to regulate the 12-36V input to 12V (and then down to 5V and/or 3.3V). Supplying a higher voltage is likely to cause damage, if not instantly than almost certainly after a while. It should in principle
be safe to connect the doorbell to a 802.3 port, if all goes as it should the port will simply not power the device. It's not really clear to me whether Cinnman connected it to a 802.3af port and had it actually power up as a result but if it does power up that's bad.
Some PoE switches (Ubiquity 8 Switch150W) can do passive 24V PoE (note that the 60W version of that swich does NOT do 24V passive PoE, only 802.3af/at). It might still be safer to use the separate injector though, if you enable one of the switches ports to passive 24V it's a bit too easy to accidentally plug in something that will get fried as a result.
I will be ordering mine soon, will take a look inside and see if I can find how it handles PoE internally.