Anything that dense has to be a black hole, because of how gravity works. Once the density is high enough for a given mass, the escape velocity exceeds the speed of light. Once that happens, nothing can escape, so it's a black hole in space. The term "black hole" is less about what it is, and more about how gravity works with something that dense and massive. The gravitational waves show us the density and mass, so that means they're black holes.