Anything can constitute a religion. Not believing in God is not technically a religion in and of itself, but when that is a major part of who you are and you take every opportunity to inform people that God doesn't exist, it's a religion.
I don't agree.
Religion may be defined as a cultural system of designated behaviors and practices, worldviews, texts, sanctified places, prophecies, ethics, or organizations, that relates humanity to supernatural, transcendental, or spiritual elements.
The absence of a belief in the supernatural or higher being would not mean that is a religion.