As is tradition, the American form adds its own ideas consistent with the national culture: submarine advertisement for coca cola, consumption that has a large affect on retail, and partisan politics.
Also, I wasn't clear if you meant it this way, but the Yule Log was already a pagan tradition, and was not added by Christians but rather adopted by them.
Merry Solstice! :)
Why wouldn't it be?
Maybe someone will design a religion or ideology that would be resistant to being used for that kind of thing even if it becomes dominant. The fact that "tolerance" is such a central idea in social justice stuff, and yet here we are today, is evidence against this being possible... but I dunno, maybe there's an approach that would work. (Flying Spaghetti Monster? Some have said that humor—beyond "mocking the enemy"—is a good antidote to fanaticism.)
Think of the term "advent" as "the arrival of a notable person or thing" — in this case, it's a thing :-)
That said, I don't see any issue in using that term here. Even if one is not religious (I'm not), they might appreciate the cultural aspects of it. Furthermore, the Call for Images[1] clearly says this in the postscript:
"QEMU Advent Calendar is a secular calendar (not religious). The idea is to create a fun experience for the QEMU community which can be shared with everyone. You don’t need to celebrate Christmas or another religious festival to participate!"