(not a designer myself btw)
Isn't design language mainly used to describe general things about how a design should work?
Take Material for example. Material itself is a design language, telling you how far apart certain click targets should be, how big text should be, stuff like that, to make a generally useable UI. It doesn't tell you what shape or what colour your button should be, that's up to the implementation, like Material UI, to decide, which is what I would call the general design.
Sure the sentence works, but now you've lost the distinction between more of an abstract concept and a concrete implementation. It wouldn't be wrong to call both material and mui a design, but in conversation it can just be useful to have a little more distinction between the two without having to go into the details explaining it.
(also damn i should have chosen a better example than material, their naming is pretty confusing)