The claim about black holes is a hypothesis based on mathematics that those favouring it will readily admit as not been proven. It is a model that seems to fit the universe we live in and explain it. That is quite different to a theist who claims that the existence of God is a fact, an absolute truth. Even if they reduced the claim to "the existence of God seems to explain much of the universe we live in" it wouldn't hold up because it simply doesn't fit our experiences and what we can measure.
This is a key difference in what scientists and theists mean when they say they "believe" something. A scientist means that having evaluated the arguments and available evidence the think it is the most likely explanation, somewhere on a scale of "this is the best hypothesis" to "it has been proven beyond reasonable doubt". A theist means that they think something is true based on faith, not any kind of rational process.