I do wish they had gone with m.2. It really feels like T2 is wasted in the Mac mini and just there to check a feature list. It doesn't even make much sense in a machine that is turned off instead of put to sleep.
T2 is Apple’s SSD controller. I suppose Apple does not trust controllers from other manufacturers and has enough data to see that with T2 the storage is sufficiently realible not to warrant an option to replace it even at own service facilities.
Here it's a bit more than just that: T2 is the secure boot system, but also the SSD controller (and a few other I/O things).
They could have put an m.2 slot though.