I am sure he is right, as far as
he goes.
The key will be whether an IOM residence will be classed (by EASA) as EU residence.
If
not, M-reg will be "third country aircraft", just like e.g. N-reg, and their pilots may have to get EASA papers, as well as maintain the original FAA/IOM ones. Unless the owner/pilot
lives in the IOM, in which case he will be fine.
If
yes then he is sitting over a fantastic loophole
IMHO, the answer is
no (because the IOM is not in the EU) so only
IOM resident owners/pilots will be protected from EASA. And probably same for Jersey/Guernsey ones.
What happens about e.g. Norway or Croatia I wonder? Not in the EU so living there is not EU resident.