I can't find any confirmation of it. It wouldn't surprise me if some Democrat in the embassy in Argentina wasn't happy about the twins being there. That person might well have said something to an ABC reporter.
It's nothing more serious than that. US embassies do not ask Americans to leave a country unless conditions are deteriorating rapidly, and then it's an advisory. It would be unheard of to take a position on specific individuals, especially the daughters of the sitting President.
Just note who on this thread is criticizing the twins for "defying" the embassy, and judge all subsequent posts accordingly.
And Margaret Truman sang beautifully.
I'd go with your guess that it was an angry leftist lifer griping to a leftist 'journalist' who was all too anxious to twist in in the worst possible light for public consumption.
And for the consumption of the gullible right here on FR, I'd say....
Since when was the State Department on our side?