An alliance implies a level of formal military and strategic coordination that I don't think is going on because I'm pretty sure it's an exclusively economic affiliation

Like if Russia or China get invaded by the U.S. I don't think the other is under any obligation to do anything about it, and I definitely don't see Brazil or India jumping in on their behalf. And India and China still have stupid border skirmishes over their dumb claims in the Himalayas

In international studies, alliance is usually used to mean military alliance. Other bilateral or multilateral alliances are referred to as cooperation, agreements, memoranda, partnerships etc. so yes definitionally NAFTA is an alliance (formal structure around coincidence of interests) you won't often hear it referred to as such. It's a usage thing not a definitiona thing, so I appreciate where you're coming from