The young earthist comparison of the distant starlight problem to the horizon problem is, quite frankly, a joke.
The two operate on completely different scales. The horizon problem concerns the laws of physics operating on the scale of the entire universe, right at the limits of what we are able to reason about and calculate, during the first fraction of a second after the Big Bang. It is the kind of question that we should expect to be unanswered and mysterious without falsifying the Big Bang, ΛCDM cosmology, or deep time. The distant starlight problem, on the other hand, concerns the laws of physics operating on scales one million times smaller, over a distance a fraction of the size of our own galaxy, well within the range that we are able to measure and reason about directly using relatively simple mathematics and rock solid, reliable measurement techniques. It’s the kind of question whose lack of an answer is so egregious that it is a complete deal breaker for a young earth.
Tu quoque arguments are already bad because they are just a way of diverting attention away from the fact that you haven’t got a leg to stand on. But if you absolutely must make tu quoque arguments anyway, please at least make ones that make sense. Comparing the horizon problem to the distant starlight problem is like comparing Mount Everest to a teaspoonful of dirt. It’s simply patent nonsense.
Actually, black body radiation is a quantitative prediction. The black body spectrum is described by a specific mathematical curve, and it is possible to determine exactly how precisely the observations match that specific mathematical curve. This is something that young earthists do time and time again: they treat precise, quantitative measurements as if they were nothing more than subjective, qualitatitve arguments.
It can do if it is precise, quantitative and tightly constrained, and the alternative explanations all make predictions that differ wildly from it—if, in fact, they make any predictions at all.
Do you know what kind of prediction really does not validate a theory? The kind that is too broad to miss. Like, for example, young earthist predictions about the strength of Uranus’s magnetic field.