IMO, motivation and outcome is what is required to label something as terrorism.
Not all unauthorized destruction is meant as terrorism, some of it is just vandalism. In my home town about 10 years ago there were several kids making dry ice bombs. Put a little dry ice into a soda bottle, put it in a mailbox, and then run away. The intent was to vandalize and be a general nuisance - even though what they were doing was overtly dangerous. They weren’t targeting any one specific person, it was random. However, had they done this same thing but only to mailboxes of POC, then it would cross over to terrorism.
Another example would be tearing down Confederate statues. Don’t consider that terrorism, if anything it is combating the constant level of terrorism that its presence exuded. But if you tore down a statue of say MLK, then it would become terrorism.
If you had drunk kids ram and knock down the stones at night in their truck, then you might chalk it up to mere vandalism. But given the nature of the destruction (explosives), and the heat they had been getting by far-right Q-Anon types, this act is clearly meant to send a message against the “NWO” and whoever they feel are part of the “NWO”. So it is an act of terrorism.
I highly highly doubt it was an RPG. If it wasn’t a shutter artifact like @anon87143080 said, it probably was muzzle flash from a rifle. It is possible they used Tannerite to blow up the stones by shooting it with a rifle to set it off. Though if that was the case, not sure why some of it was left on scene, apparently. They would have just followed up with a 2nd round.
Dynamite could have been used if someone had access to it from construction or other applications. IIRC with dynamite you have the dynamite, blasting caps, and then wire to run from the detonator to the explosives. Then you need an electric charge via the detonator to send down the wire to set off the caps to set off the dynamite. But I don’t think that would have resulted in a flash like we saw. But it would explain if they didn’t do it right why there would have been explosives still on scene if bundle of sticks did not go off.
There was also a puff of dust between the flash and the explosion. Which could possibly be from the muzzle again kicking up dust, or IIRC watching Mythbusters, the explosion will cause a wave to travel back up the wire and kick up dust.
Anyway - I am sure the ATF/FBI/GBI will figure that out.