Interesting, and worthy of consideration alongside all other speculation.
What leads me to think it was NOT a European hoard is that hiding/missing this quantity of DEs for decades would have been tough. Plus, you had representatives of U.S. dealers actively scouring the European banks in the 1950', 1960's, and 1970's. I would think they wouldn't have missed a 20,000 coin hoard all that time.
That said, clearly there are sensitive issues with disclosing the COMPLETE story with these coins. I hope Mr. Gillio does release them at some time, even if posthumously for himself or the counterparties. I suspect that whomever he purchased them from was sensitive about the sale for whatever reasons.