So is there an actual threshold and actual requirements that make it clear who is or who isn't a member? Because otherwise, it just sounds like a No True Scotsman.
You're talking about groups that are organised locally and are subject to their own specific organisational tendencies. Antifa are not a monolith. I know of orgs that are countered by other orgs for being violent. I've been in situations where people police who is and who isn't part of a demo, and if people think someone could cause shit. You don't get some card or anything, you go to planning meetings, which are usually word of mouth, and if people there think you're cool, then that's the vetting process that you get. There's no formal membership and if people think you're only there to stir up shit you will be swiftly excluded,
from my experience. Antifa function on the idea that there's general solidarity in the protest, and that anyone is welcome to join into the bloc, but they are, to a fault, expecting that malicious agents hoping to undermine them won't do it out of intimidation.
The key point to drive home is that antifa is not a monolith, it's a loose methodology of organisation where every group is independent. They're not chapters run by a super-group. They're amorphous by nature, and that can include all the way up to what happens in the moment. To bring this back to hammerman, it's not impossible that he's antifa, but it's highly improbable considering how he went in by himself, and refused to do anything with protestors, even edgelords would go around passing bricks or yelling at people to join in (if they for some reason would want to attack an autozone). If he identifies as such, without there being any evidence to tie him to a group, then it's about as useful as saying that he representes the Soviet Red Army for all the difference it makes.