If this were a second floor of the was you still say that it's the Association responsibility?
Yes.
The upper boundary of a lower unit would be the surface of the ceiling.
If you live above me, my upper boundary would be the ceiling, your lower boundary would be the floor. What's in between, the joist, piping and electrical equipment would belong to the association. Same is true for walls.
I once declined a charge from condo association for snaking out a kitchen drain. Drain was shared with unit next door (kitchens were back to back).
While my tenant probably did stop up the drain, no one could really know.
Rather than argue about who was responsible, I just referenced the paragraph in the master deed that said my property stopped at the surface of the shared wall. In other words its your pipe and its your blockage.