It's as if someone stood up and said, "Jesus was born of a virgin, ministered, was tried, crucified and rose again, and salvation through him is the only hope for humanity and we will all be better off if everyone can see that" and you said, "Well, he didn't say he's a Christian."
These folks have a path that they believe is the only hope for the human race and they believe all other views should be eliminated. That's not only a religion, that's way more hardcore than most religions.
Let's put aside the idea of religion, though. Do you really have any doubt that these particular individuals ascribe to the idea that "natural science has authority over all other interpretations of life, such as philosophical, religious, mythical, spiritual, or humanistic explanations?"
What't the practical difference between "scientism" and "heretics"?