> translating C's semantics into those of a related language with arbitrary but technically defined behavior instead of having UB

There is no “instead” here. A C implementation defining behavior for certain UB still falls under what a C implementation is allowed to do for UB.

I don’t know if Fil-C is a fully conforming C implementation, but it could well be. In what way is it nonconforming that would warrant describing it as implementing “a related language” and not C?