I think the FSB theory is even less credible than the "random drunks" theory. If you're Putin, you don't really want to make your statement in Moore County, North Carolina, and you're sure not going to destabilize the US by shutting down Moore County. And Putin loves sending messages. If he'd ordered it, it would have been intentionally obvious, and then he would immediately have issued an equally obvious bullshit denial.
As for the MAGA/Jesus types hitting the grid, I'm not saying they did, because there are plenty of other possibilities.
But you could imagine that they might have wanted to shut down the show from far enough away that they could avoid being noticed. They might have feared they'd be seen and even recognized if they came close to the building, in the middle of town with a lot of people around and at least some of those people on their guard against them personally.
And/or, as long as I'm making stuff up wildly, I could imagine they might have had military training that (1) suggested substations as targets, because they're effective military targets, and (2) didn't deal with the risk of getting caught by law enforcement, because the FBI doesn't typically isn't methodically tracking you in war zones. If they followed that training without independent thought-- and independent thought isn't one of their well known strong points-- then they might have done exactly what they did. They might not even have really been clear about how LONG they were going to shut down the power. It's not what they would have done if they'd been trained as INFILTRATORS, of course.
Or I can spin even more random stories. They could have wanted to do something bigger than the theater to make a point, or a threat, or even make other people believe that the judgement of God truly was coming down. Their Fearless Leader tweeted something that was technically not an admission and technically didn't claim "credit"... but sure kind of sounded like an admission and a claim of credit. Then she walked it back with "The Lord works in mysterious ways", which was just KIND of plausible because she's a known religious nut and what she'd tweeted just happened to also admit that interpretation. You could imagine that that whole show was designed to create fear of crossing her and her group, but in a plausibly deniable way. Almost Putin-like, but aimed more to create uncertainty and less to assert power by daring others to challenge the lie. She WAS a PSYOPS officer.
Of course, to come up with that plan, she'd have to be totally ignorant of how thoroughly law enforcement can crawl up your ass when it's a priority for them... but law enforcement's full capabilities are something she COULD plausibly be ignorant about.
Point being we don't know, and it could have been anybody, and I could make up these stories all night long, but if I were the cops I'd be keeping those people in mind as suspects (while going over the whole place with a fine-toothed comb). And I don't think I'd be suspecting the FSB unless something fell in my lap to point to them.