I will try to explain in other words. Given a formal theory of absolute geometry, we are able to write the expression whose meaning is the paralel axiom.
The question is, that expression is a "proposition"? AFAIK a "proposition" in propositional logic is a "thing" that is true or false. It cannot be undecided.
On the other side, we can add this expression as an axiom and we will have euclidean geometry.
Now I guess there is no issue to consider that these expression is a proposition.
Probably I need some precise definitions of what a proposition is, what a predicate is, what an axiom is, how a language is defined, ...