It is a very extreme way (binding adoption of EU rules) of achieving a legitimate goal (preventing UK subsidies from exploiting EU market openness).
However, provisions like this would be different because they would bind the UK to follow rules that don't currently exist, and that would be made without their input or clearance.
Huge ask.
1. It's offering more access to its market than it offers any developed country, including Canada or Japan;
2. The UK is closer and a much larger trading partner;
3. Past UK behavior is no predictor of future policy on subsidies.
4. It can.
If that proves impossible, then either accept the EU demand, scale down the deal's ambition on market access (leave some tariffs in place), or scrap it entirely.
It looks doubtful they'll be enough alone, however.
Just a theory.
/end