That's the other possibility. Someone needs to test it.
When my base got hit back in pre-com beta, I had to load 3 coal into each brazier the next morning. If it's per "activation", walking off the claim definitely causes them to re-activate without needing the claim aggro timer resetting.
I think coal usage makes more sense in that it prevents people from popping on/off the claim just to drain the braziers of charcoal.