Other fun arguments in the same vein:
Is atheism a religion?
Is not playing golf a sport?
For extra fun, try explaining the answers to both in a non-contradictory way.
How are those the same? You need to define "religion" and "sport" rigorously first.
Since you haven't provided one, I'll just use the first sentence on the wiki page:
Religion is a range of social-cultural systems, including designated behaviors and practices, morals, beliefs, worldviews, texts, sanctified places, prophecies, ethics, or organizations, that generally relate humanity to supernatural, transcendental, and spiritual elements.
"Atheism," without being more specific, is simply the absence of a belief in a deity. It does not prescribe any required behaviors, practices, morals, worldviews, texts, sanctity of places or people, ethics, or organizations. The only tenuous angle is "belief," but atheism doesn't require a positive belief in no gods, simply the absence of a belief in any deities. Even if you are talking about strong atheism ("I believe there are no deities"), that belief is by definition not relating humanity to any supernatural, transcendental, or spiritual element. It is no more religious a belief than "avocado tastes bad." If atheism broadly counts as a religion, then your definition of "religion" may as well be "an opinion about anything" and it loses all meaning.
If you want to talk about specific organizations such as The Satanic Temple, then those organizations do prescribe ethics, morals, worldviews, behaviors, and have "sanctified" places. Even though they still are specifically not supernatural, enough other boxes are checked that I would agree TST is a religion.
I have no idea what you're on about with not golfing being a sport.
How are those the same? You need to define "religion" and "sport" rigorously first.
This is really the crux of the argument. There are no absolute authorities on religion, sport, or in the case of the original post, mathematics. We can have definitions by general consensus, but they are rarely universal and thus it's easy to cherry pick a definition that supports any particular argument with no ability to appeal to authority.
I have no idea what you're on about with not golfing being a sport.
It's mostly a troll argument, but you can easily trip up people with interchanging the definition of "sport" as a thing ("golf is a sport") or an activity ("playing golf is a sport"). Then after trying to hammer down the definition more exactly, you can often poke holes in it with more questions like is chess a sport? Is playing Counter Strike a sport? Is competitive crocheting a sport? All of these ambiguities are possible because of the lack of a universal authority in the realm of sports, though some people try to pick an authority such as the Olympics to prove their point.
I'd argue that atheism is a feature of a belief system and that the system may or may not be a religion. There are religions that don't feature a belief in any gods. Similarly, your personal belief system may not be a full blown religion, even if you did happen to be theistic.