As an astronomer, I get to ignore the details of the things that we dont understand. Theres a lot of work that we can do on scales that we do understand, and there is actually a finite size that I can associate with a super massive black hole.
Proof of the black hole is a tremendous amount of mass inside a very small volume. There's 4 million times the mass of our sun within a region that's comparable to the size of our solar system.
As an astronomer, I get to ignore the details of the things that we don't understand. There's a lot of work that we can do on scales that we do understand, and there is actually a finite size that I can associate with a super massive black hole.