Dithering would be an actual technical technique with a similar end effect, but it's not about adding grain per se, nor is this made through dithering.
And I think the guy looks for a design term for the visual style (similar to how we say "flat", "skeuomorphic", "italian style"), not the technical name of how it can be achieved, anyway.
For that, I'd say, both the geometrical shapes in his design and the added grain, bring to mind the so-called "memphis style".
I agree and the grain/transparency effect specifically looks like the gradients made using a (physical) airbrush that show up a lot in the Memphis style.
And I think the guy looks for a design term for the visual style (similar to how we say "flat", "skeuomorphic", "italian style"), not the technical name of how it can be achieved, anyway.
For that, I'd say, both the geometrical shapes in his design and the added grain, bring to mind the so-called "memphis style".