It would be hypocritical of an atheist to celebrate Christmas so they do nothing untoward by their non participation or what you describe as 'dismissing' of it.

In reality, given that you have focused on the religious element, the vast majority of people who engage in Christmas activity are not religious in other than the most superficial of ways and many are not even Christians.

It is also of note that there is no definitive evidence of the year of Jesus's birth, let alone of the exact day.

Take care. Stay safe. ☮️