Expats are typically people living outside their country on a non-permanent basis (on assignment or some temporary basis), with no intention to permanent emigrate to the country.
Immigrants however are people who move to a country with the intention of settling there permanently.
From the article's description, it sounds like these folks are expats, probably in Portugal on a D7 or Golden Visa (both are temporary visas). Some may elect to apply for citizenship after a period of residency (5 years), in which case they would become immigrants if the choose to remain. I guess some just stay long enough to get an EU passport, and then they buzz off -- then they wouldn't be migrants.