Bitcoin Core uses BIP-386 for tr(...) descriptors, and in the case of tr(KEY), the produced key is tweaked according to BIP-341:
In the current descriptors.md, it is documented that tr(KEY) uses KEY as the internal key, with no further comment:
By looking only at descriptors.md without knowledge or reference to BIP-386, one can interpret tr(KEY) to use KEY directly without tweaking.
Ideally the docs should be expanded to reference BIP-386.