At no point was it ever specified what the conditions to be considered participating are.
However attacking the relic is not something that can be counted.
You are also not actually participating in the point of the event if all you're doing is attacking the relic anyway.
The afk system isn't specific. It works the same no matter where you are. Instance or not. So if you're only using 1 skill and not moving or doing anything else then no you are not actively participating. As even just moving your camera is enough to stop afk timer.
The fact it marked you as AFK and applied an arena barred debuff on you proves its working exactly how it should be working
So basically to "participate" I need to die and move my camera (movement of character completely optional) and I will get reward. That's why I think the system is broken.
And for conditions - I tested it, few times I had Halcy without reward so I now exactly what I need to do in order to get reward - Kill, Assist, Die or attack guards (didn't test this one, I like my rewards). Basically I need single action that give progress points to me or enemy. Then I can be AFK if I don't care for debuff and I will still get reward or move camera to not have debuff and go to another Halcy to do nothing.
To sum it up with colorful example. Person that do nothing or worse because they give score to the enemy is worth more and get rewards than person that buff team with skills and songs, heal, actively participate in PVP, go help destroy guards and relic and luckily won't die during all this, but for some reason don't get any assist (yeah low GS... no assist for you), kills or even progress points, then that person will be punished for "not participating". I have been there and it is happening.
Edit: And lets not forget that they can get AFK debuff while attacking relic, which would prevent them from participating in next Halcy... but their contribution was non existent anyway according to points...
See the issue?
I don't expect this to be solved immediately next patch or something, but just not going immediately into defensive position (because that is mostly what I see from responses - everything works as designed and we won't change anything) the acknowledgement of the issue and info that "maybe someday, when the times allow for it" would be nice.
And for AFK debuff... if it works as designed I can move camera while attacking... or I can give up and spin in place at spawn point, that should also work fine.