My complaint with 'Christians stole Christmas from the pagans' has less to do with keeping Christianity special, especially since I'm not a Christian. And every culture has cultural drift and Christians are no different.
It comes more from criticism of people like Joseph Campbell and monomything. Which tends to flatten actual cultural drift and abhors that similar ideas can rise independently due to common experiences. Especially when it just exists for some conspiratorial jab (ala Zeitgeist) or some gotcha to Christians. When people talk about Christmas trees, Saturnalia, or equinox rituals in general, it's often not coming from a place of authentic scholarship but just really superficial pop-takes.
To me it's the equivalent of saying the global flood was real because there's flood mythologies in every civilization. It's reductive, it doesn't look at how civilizations arise in flood plains, so of course they'll have some common myths about flooding. Same is true of how things became rituals revolving around specific seasons. Spring is going to be about new beginnings, fall is going to be about harvest, winter is going to be about thankfulness, meditation, using up final perishable stores, longing for light in the darkness, etc etc etc. The only places that you don't really see a lot of similar seasonal festivals are places on the planet that don't have highly visible seasons.
Well things are culturally drifting away from Christianity and back to where it should be then, in Natural cycles.
I don't have the mental capacity to actually go over the scholarship you say is only a jab at Christianity (it's not).