Spec is at heart a data modelling system. So for example,

  (s/def ::integer integer?)

is very explicitly not a definition of an "integer" type. It is preparation to model some data x and check that the data will evaluate to true if someone uses the predicate (integer? x). However, this is obviously enough to implement a type system.

But you can also do some very loose things with spec that aren't done in a type system (I speak with a cheerful ignorance of type systems). Eg, I can spec a function so that one argument is smaller than the second. This allows my testing system to automatically generate the correct data to test my function.

So spec can used to implement a type system but also to implement things that aren't type systems.