The cosmological constant was a postulate before it was tested. Even if they're constructed from 'first principles' (as if such a thing existed; a Schroedinger wave equation is a good example of this, however) it still has to be validated by experiment.
Furthermore, given that the cosmological constant was rejected soon after it was suggested (Einstein called it his biggest mistake, in fact), I doubt it could have had the status of 'proven fact' either before or after Einstein.
Only recently is it coming back into vogue, but as I said, cosmology has the capability to be really all over the place.