I concur with casey's answer on the linked question. There are two basic facts:
The FARs say what a pilot can or cannot do.
The FARs say a pilot must comply with an ATC instruction.
And then a complication:
ATC might instruct a pilot to do something specifically disallowed by the FARs (for example, exceed 250 KIAS below 10000 MSL).
Really what it comes ...