A quick review of the parent comment would tell you that it never rises to the level of being a hypothesis because it is rooted in category errors and false distinctions that make it neither testable or falsifiable. For example, the appeal to comparison of immigrants as a bloc with shifting American fictions about what constitutes membership in its racial categories. These boundaries are too ill-defined and overlapping with each other to be used in this fashion. What is the process by which one transitions from being an immigrant to one of these groups and how would you observe and control for how any of these (unstable) requirements are correlated to test performance? And of course the same appeal Simpson’s paradox would also have to be applied to the category of immigrant itself before it could ever be considered in this way, since this umbrella contains even more and diverse members than the racial categories with which it is contrasted.
That's nonsense, it's clearly (obviously!) possible that the math score of a country decreases because of immigrants having lower test scores and immigrant share increasing. Which makes it a valid hypothesis. Any problems with definitions are orthogonal to that.
If you believe definitions constituent to a hypothesis are ever orthogonal to it, this would be a mark in the ledger in favor of there being an overall decline in reasoning ability. By this logic, I can simply say “x is a responsible for a decline in y, as evident by the comparable measure of z” without ever having to define any of those variables and always be correct. Does that seem rational or useful to you?
Look, I can understand what you say, and you can understand me, without us defining all or any of the words we use. We don't need a definition of definition. We can know what things mean without having a definition of "knowledge" or "meaning".
That more abstract claim was never what is at dispute. The OP’s claim instead requires clear and consistent definitions of the categories involved in order to be a falsifiable hypothesis, which were not provided. Your defense of this is apparently that because it also exists on the low end of the spectrum of some basic intelligibility that also makes it a hypothesis, which is, again, also incorrect. It’s a supposition that never rises to this level because it lacks these aspects.
At most hypothesis may be false, or unsupported by the evidence, but never is a hypothesis by itself "bigotry".