I think the hypothesis is not that English-speakers in other countries didn’t drink on Christmas, but that doing so wasn’t ever prohibited in other places. Whereas in America, Christmas was banned (in places, by the Puritans) for being too secular and merry a holiday and in reclaiming the holiday, people leaned into the “merry” aspect.
But i think, “a popular poet used it and it caught on” is at least as likely.
In the UK, “merry”, despite the 16th-century carol, “We wish you a merry Christmas” and Dickens, the word became more notorious with rowdiness and drinking, with “happy” being more refined, and indeed QEII and other royals stuck with “happy.”
Meanwhile, in the USA, there was “A Visit from St. Nicholas” (“Twas the night before Christmas…”) and if “merry” or “merrymaking” involved boisterous carousing and drinking, no problem was seen with that.
The Irish and British have Halloween now, so the USA can take credit for keeping “merry” in the under-the-bridge Oxford dictionary (not really, if Dickens used the word), as well as Pagan festivals.
Do you wish people a merry Halloween? In the US it’s always Happy Halloween. Heck, that’s alliteration.
I ain’t eating that “merry” guy’s chocolate, too weird.
There are a couple of much more archaic uses of the word. Robin Hood’s Merry Men, Merry-go-round, and the merry month of May.
You’ve always had it. We stole it from you, poured America all over it, and it looks like you’re slowly warming up to the “new” version.