JSRG:
around the time paganism was an actual thing
I believe we are using different definitions of paganism. In this instance I’m referencing any religious practice or ceremony outside of Christianity. Therefor, “paganism” has always existed.
Normally “pagan” refers to the pre-Christian polytheistic religions like the Greek/Roman religions, Egyptian religions, etc. The only time I’ve seen someone use your definition, or something close to it, as Mortimer J. Adler in his book “How to Think About God: A Guide for the 20th Century Pagan” but he made a point of clearly giving a definition for his usage of the word ‘pagan’ to avoid any confusion.
is it based on pagan religious practices?
Yes. The use of evergreens inside your home is based on a pagan religious practice.
And what is the proof of this? You can’t simply say “pagans used evergreens” (assuming that is the case) you have to show an actual link that it was based on it. Or, at the very least, account for the 1000+ year gap between the usage of it by pagans and the usage of it by Christians. To claim it was based on the pagan usage requires the belief that about 1000 years after it was abandoned, some Christians decided to look into history and dredge up some old pagan practices for no particular reason. That appears highly unlikely. Certainly no proof of such has been offered. Thus, any similarity would appear coincidental.
More plausibly, if evergreens were used by pagans around the winter time, it was used because they are a common tree that keeps its leaves in the winter. And with Christmas being in the winter, if you were going to use any trees or plants, you’ll want to use the ones that don’t end up looking barren during the winter.
The most likely explanation for any alleged similarity between pagan festival usage of evergreen trees and Christian use of them is that the pagans used it for convenience sake, then it was forgotten about for 1000+ years, then Christians (completely independent of prior pagan usage) started doing it because, again, it as the tree that didn’t lose its leaves in the winter. Thus, any prior pagan usage of such would be, in the absence of any evidence demonstrating an actual connection, coincidental.