Does that mean that no EU country needs to implement it?
Yes, in the sense it's automatically implemented when it becomes EU law. The Aircrew Regulation is already law binding in the UK, for example. If the ANO hasn't been amended to implement it, that doesn't matter, in the UK the EU law takes precedence over it.
IMHO the issue will never be enforcement (because "operator residence" etc etc etc will never be possible to establish in the context of a realistic ramp check) but insurance
No, the "issue" will be that people generally don't break laws. This rule is highly enforceable. For every special case we conjecture on the forum of some bloke born in Mongolia who spends 5.9mths of the year living on the international date line and 6.1mths flying in the EU blah blah blah, the reality is that the people impacted are, on the whole, EU resident EU passport holding owners of FRA. So you land somewhere and are ramp checked. You are flying an N-reg, so you must have your passport with you. It's a UK one. OK, you can claim you are resident outside the EU. Do you have a single document to prove that? Your FAA certificate shows your UK address, this is of no help. OK, so they make you sign a document that you claim residency outside of the UK with small text that it is a criminal offence to knowingly make a false declaration. Who exactly is going to do that? No-one I know. And even if someone does make a false claim of this sort, and gets a letter asking them to forward evidence of their residency, what are they going to do? So you forge some documents. Perhaps the investigating agency will forward these on. If you are resident in the US, you are liable to US tax. Good luck to anyone explaining to the IRS that you were only falsely claiming US residency for the purposes of evading EU aircrew regulation.
You always make this enforceability point on every thread on the N-reg. You must think it has some signficance. I don't see it.