Probably because they don’t show real time share prices.
And this statement doesn’t necessarily preclude them from preventing trades that would be executed at a price far different to the one shown in the app.
Given the number of complaints from people trying to buy a stock and having to pay far more than the price shown that used to be on these forums. And now that these seem to have dwindled and, to a degree, been replaced by complaints about rejected trades, I’d say this is a reasonable theory.
I agree correlation doesn’t equal causation, but correlation often provides a basis for investigating causation - hence this post.