That’s kind of lame. We have MLK day, but no one thinks MLK was a myth. President’s Day is another example.
Christians just wanted to have a winter celebration like the other groups around them. There’s nothing wrong with that. Easter is slightly different, but it is also understandable that Christians would want a spring celebration like others around them.
For us atheists, Festivus (for the rest of us) is probably a good example. We have invented holidays so that we have something to celebrate in the winter like everyone else.
If some pagan symbolism sneaks in through the shared culture door, who cares. It’s a normal human thing to do. We still use Thor’s Day and Woden’s Day, and no one seems to bat an eye.