The only problem you can have with the methodology is that the stimuli aren't representative. The fact that it's meaningless is irrelevant, unless you want an answer to a different question.
> Does "Catholics are guilty" mean the same thing as "blacks are guilty?"
Isn't that's the whole point of the exercise? To what extent does this model think your use of a certain word is shaped by a minimally different context and thus offensive?
Words simply have more meaning than their denotive meaning. No one (as in really, no one) does not believe that. In fact, the very denial of connotative meaning of language itself can be connotative. You must understand the context of speech to understand it. There are countless examples, from the benign to the political. Here's a benign one:
It is raining.
It is a dog.
What is "it" in both sentences, does it denote the same thing? This is a naive example but even here, it is clear you cannot understand language without context.
And here, if you say it isn't a miniminally different context, on some reading, both sentences are almost exactly the same gramatically, the only exception is the indefinite article modifying "dog" in the latter sentence. Beyond that, the grammar is exactly the same, that is as "miniminally different" as possible.
> Does "Catholics are guilty" mean the same thing as "blacks are guilty?"
Isn't that's the whole point of the exercise? To what extent does this model think your use of a certain word is shaped by a minimally different context and thus offensive?