I agree that the "eligible attacking units" doesn't seem consistent since you declare the target of attack (and after that you're in combat) before attackers are chosen.

The way I would interpret that is that you cannot declare an attack without units that could attack. So the various Shrine combos that use non-combat damage (e.g. Pestilence, Sorcerer) won't work unless you have a unit in the battlefield that can attack. However, I would say it is still legal to declare an attack, pick the zone, then play Pestilence (or whatever) but choose not to actually have a unit attack even if it can.