Take this with a grain of salt as I've never touched NixOS mobile and thus have obviously not tested any of this.
I think the solution is change the device = "oneplus-enchilada"; to device = "oneplus-fajita"; on line 29 of flake.nix
Theoretically, the rest of the guide should work.
It looks like the 6T will use the same firmware at the regular OnePlus 6