I think the more relevant bit of the report was:
The AAIB sought an opinion from the CAA as to
whether the pilot’s licence was valid for this flight with
this aircraft. As this N-registered aircraft was being
flown under the privileges of an FAA licence, the CAA
asked the FAA for clarification but the FAA did not
provide a definitive answer.
So it's an aerial work operation in the UK by an operator based in the UK, but flown with an N-reg. And when the CAA, correctly, seeks, not even to enforce regulation but to clarify the regulatory position by the ICAO route, the FAA says "Eh, whatever...".