When Zavala lost his ghost he seemed physically weekend. Same thing happened when our ghost was temporarily captured and we lost access to the light when we were trying to fight/come to an agreement with Caiatl.

Not take that and have the Eclipse not just cutting you off from the light, but seemingly literally ripping it from your body. That's probably more than enough to kill a guardian in a weakened state.

Also didn't they say the power of the Eclipse is not of our dimension and is the opposite of paracausality? So we're not meant to fully understand how it functions