International history is filled with examples like this. The history of the Russian gulags is probably the most stark example, they were actually pretty decent (comparably) before everything outside went to shit…

Correlation vs. causation.

A lot of things which impact “intelligence” can also have varying impacts on speech/articulation/communication skills. However it is important to note that the correlation isn’t strongly positive or negative (IE savant syndrome).

So while some disorders may have some common correlations with specific speech/articulation patterns they (usually) are not good diagnostic criteria and extremely broad generalizations like the one above are particularly bad.