Amongst other things, don’t you have to say it b’lashon neder or b’lashon shvuah? I’m far from an expert in the topic, but I did have a sheilah in this area once, and I’m almost positive that’s what I was told.
I also thought that if it was clearly a joke, it wouldn’t be an issue. If it would be, most people would be in trouble.
Also, if it doesn’t have to be b’lashon neder, we would all be “over” on it all the time. “I’ll be there in 5 minutes.” “I’ll see you tomorrow.” etc.
Of course, there is probably a “maaleh’ to being extremely careful with all of one’s words, but you don’t give someone “mussar” for not being on a higher level (especially if almost no one in the world is on that level) as long as they are not performing an outright clear-cut issur.
Granted, the Gedolim are probably on that level, and I can understand if you thought that DY is one of the Gedolim and hence, should be held to a higher standard than the rest of us. THAT was probably what you meant to say :)!