What are they then?

Nobody knows, so it’s premature in the extreme for you to start describing this as an act of terrorism.

We have literally no idea why these people let that stuff off. It’s not an obvious example of terrorism - no-one is blowing anything up, or shooting, stabbing or running over anyone, there is no counter-terror response. It’s not like a failed suicide bombing. attempted assassination or rampage - it’s a release of a chemical irritant.

This might be two of the least effective terrorists of all time. It might equally be an accidental discharge of a canister someone was keeping for self-defence, or someone’s idea of a joke. At this stage reserving judgement seems prudent, to me at least.