Well I can't say snopes proved this to be a hoax. A bunch of circumstantial hypothesizing is all they presented to attempt to discredit Washington's Vision.
Regarding your point on the term 'Union'; I don't think the vocabulary of supernatural entities is bound by the contemporary language of men. I guess I would expect that if an angel used specific terms then that might be important to note and remember regardless if it was in use at the time.