I can try to find out more, but I won't be seeing him for at least another month. He was very cagey with the details, because he used the process (of living in PR doing nothing) to drop his yearly average earnings below the threshold for renouncing citizenship without penalty. If the US authorities discovered this, they might retroactively assess penalties or fees of some sort -- I wasn't clear on exactly what he was afraid of.
I got the impression he already had an EU passport before buying citizenship in SKN and renouncing his US citizenship (which I think he did in the Bahamas). He had almost no personal assets, everything owned by layers of companies, to prevent his ex-wife from getting any of it. Which was another reason he was really cagey about details.
So, I can easily believe he left something out of the process.
Also, this was all pre-pandemic. So even if there was a loophole then, it might be closed now.