It sounds like you have a photo of the toilet paper holder hanging off the wall? Is it time and date stamped? You could send this to Airbnb as proof that it was like that when you arrived if it comes to that.
Is the host asking you to pay for the plumber? If you truly didn't cause the damage, then just decline to pay, and reiterate that it was like that when you arrived.
And I can appreciate that you didn't want to be "that guy", the guest who has complaints from the moment they arrive- of course hosts aren't fond of those kinds of guests. But if you arrive to find things broken, or not functioning properly, or missing, it's best, as you found out, to let the host know ASAP. It's also the accepted Airbnb protocol if something isn't right- you are always supposed to let the host know, giving them a opportunity to correct the issue, rather than just leaving a bad review or involving Airbnb right off the bat. Hosts get upset when guests complain about stuff in the reviews that they never even let the host know about during the stay, because in most cases, the host could have done something about it if they had only known.
Reporting broken things or missing amenities aren't necessarily complaints, just ensuring you aren't blamed for something. "Just wanted to let you know that the when I arrived, I found the TP holder hanging half off the wall. No biggy, it won't affect my stay, just letting you know it wasn't my doing."
I had a seasoned Airbnb guest call me up to her room a few minutes after she checked in to point out that a shelf in the closet was loose. She wasn't lodging a complaint, she just didn't want to be held responsible, which I thought was a smart idea for her to have done. (I actually knew it was loose, I'd just forgotten to get the screwdriver and tighten it up after I finished cleaning, and it's not the sort of thing I'd charge a guest for anyway, even if they had accidentally done it)
Something like a clogged toilet is really important to let a host know about, because if it overflows, it could cause extensive damage to subfloors and even the ceiling of a downstairs unit.
@Tater1