Uni-typed is actually a very specific technical term. Mathematically you can prove type theorems about languages regardless of whether they implement types as language features. Dynamic languages are thus modelable like this and when you do so the first approximation is to say that a dynamic language is a language with a single type (uni-typed).

The reason this is valuable is it opens up directly the techniques for "hybrid typing" such as PHP->Hack and Javascript->Typescript. It also is the basis for the compiler using type inference to accelerate certain portions of the code.