That doesn't match my experience with the usage of a labeling or prefatory colon (continuing with my ad hoc terminology). I'd read it as

Here's an x: X.

– which is nonsensical (the type X is not an example of the instance x; indeed, it's the other way round), or as

It's the case that x: more detailed things that explain or follow from x are X.

– which is also backward (since a type is a more general thing than one of many specific instances of the type), or as

Here's a quick premise x: the main point is X.

– which is wrong because x is more relevant (because it's a local variable, whereas X is likely global and shows up in many places; indeed, within a particular scope many variables may simultaneously have the same type, but a name refers unambiguously to a particular variable), or as

Label x: thing X.

– which is wrong because x is not a name for X.