They were actually using aerial mortars in Portland three weeks ago in the riots there. They were firing them at the courthouse.
You need a federal explosives license to get your hands on commercial grade fireworks. The requirements include a location to store fireworks before they are used and interviews from BATFE agents before you can get a license. I'm pretty sure BLM would not not qualify to say the least as I don't they could convince anyone they are putting on legitimate fireworks shows.
I read the article and it just says "aerial mortars". That doesn't really mean anything. At this point unless we see evidence then it's safe to say these are just consumer fireworks being used for vandalism.
What I don't think people understand here is fireworks used in public displays are "explosives" and they are very controlled for obvious reasons. I'm sure some slip through the cracks here and there but it's quite rare I'm sure.