Well, assuming by isolated to mean completely and utterly cut off, no contact at all, no spying or scrying, they would likely assume that:
1: The other civilization, being obviously inferior to their own, has not advanced at all.
2: The other civilization has developed in a manner similar to their own, because that's the only logical way to develop of course.
3: The other civilization has advanced in a manner consistent with the path they were taking at the last point of contact. Anything which the civilization considers impossible or unimaginable will not be predicted; after all, why would they have anything as ridiculous as that? If both civilizations were nearly the same at the last point of contact, see 2.
No matter what they do, due to the large degree of diversion and the span of isolation, there will be no way to accurately estimate the development of the other civilization without gathering up-to-date information.
And yes, one century is enough to advance to the point of being unrecognizable. Two words: Industrial Revolution. From muzzle loaders and horses to airplanes and nukes, 1845-1945 respectively.