My townhouse bottom floor flooded with sewage pipe water this past weekend. It took calling in a specialist plumber to fix the issue. I asked my landlords when they were going to bring in someone to clean the place up. They said given my lease, I was responsible: "Yet the reason the house was flooded was because the drain was blocked/clogged by stuff flushed down the pipe. It's tenant's responsibility to not to clog the drain according to the lease"

I asked what caused the blockage and was told: "The plumber got a lot of paper residue the first day. The second day, since it is deep down in the pipe, the plumber push whatever object in the pipe down to the main pipe instead of pull it out of the toilet, so we don't know specifically what blocked the drainage. Our guess is that it has to be bigger and more solid than paper to cause that level of blockage, but we didn't see."

So my question is, is there a definition of a drain that is standard out there, or rather of where a 'drain' ends? My lease is fairly boilerplate. But I feel like if the blockage was so far down that you had to bring in a specialist, that neither the normal or master plumber could fix the issue, and that you had to as least mark outside where the water lines were (even if it appears no digging occurred) how can I be held responsible for that? If I can't see if and there are no warning signs that I know of, and you are near the main pipes, is it still the drain? Especially if they don't know what cause the clog (I think previous tenants likely flushed something that created a partial blocked the pipe and I got the side effects or that).

Which goes back to, is there a standard definition of a drain for a typical renter's lease? Admittedly they haven't come after me to pay for the plumbers or anything, but still, I think they should have to foot the bill for the clean up.

And maybe additionally, if some other tenant really caused the issue because they didn't honor the lease and my landlords never did anything about it, is there any way to push on that?

edit: clarified the question slightly, added another.