FWIW, this is the first time in the past decade that my boss hasn’t gone on a rant about the (imaginary) War on Christmas.
Likewise the idea that “Xmas” is somehow an attempt to eradicate Christ from Christmas. Never mind that the early Greek Christians used “X” (or rather “chi”) as a shorthand for Christ, nor that the formulation “Xmas” can be seen as early as the 15the century. Nope - it must be those darned PC people again.
ETA: Waitaminute… wasn’t Irving Berlin (who wrote “Happy Holiday”) Jewish? A-HA! CONNECT THE DOTS, SHEEPLE!
‘Merry Christmas!’
‘All holidays matter!’
It’s way older than that. However, the pressure to use it in lieu of ‘Merry Christmas’ because the latter is viewed as an imposition, is newer. Not brand new, but not anywhere near as old.
I’m not big on the Christmas war thing either way but I really don’t get how the left/secular side of it (for lack of a better term) can claim it’s imaginary. Non-Christian/non-religious in recent decades are much more likely to complain about Christmas per se, centric celebration than in 1942, for sure more than I was a kid (60’s) when it was just getting going. Again in those days, I heard it directly lots and lots of times, ‘Happy Holidays’ was pushed in lieu of ‘Merry Christmas’, in NY, where I grew up, because of Jewish sensibilities, where a personalized greeting of Happy Hanukkah was not practical or too long to do both, same with complaint about Christmas carol singing at school. I really was not imagining that. As time has gone on and other people who view Christo-centrism, if you will, as offensive or annoying have grown in number nationally, it has intensified nationally. Holiday tree, Holiday party, and now the follow on phase as in OP’s links, he didn’t make up, of complaining Santa or even ‘Holiday’ is an offensive expression of religion. That was not the sensibility in 1942 or 1965, just wasn’t.
Again, is this the biggest deal in the world? Not to me. Is there an argument to make the public square less Christian-oriented in sections of the country (and now whole country to some degree) that are less so? To some degree probably IMO. But the idea the movement against Christmas-centrism is ‘imaginary’ bugs me, like most things which are obvious BS. 
That’s always how I have understood it, going back to the 1980s/1990s. “Merry Christmas and a Happy New Year” is too long so “Happy Holidays” was what was said. I’ve never understood the “War on Christmas” crowd. There is another big holiday that is always exactly one week later!