Can some one please let me know I understand the below solution correctly?

Carbon-14 dating reveals that the megalithic monuments in Brittany are nearly 2,000 years as old as any of their supposed Mediterranean predecessors.

A. as old as any of their supposed
B. older than any of their supposed
C. as old as their supposed
D. older than any of their supposedly
E. as old as their supposedly

Though option B and D are both gramatically correct, The focus in the sentence is on the fact that the megalithic monuments are older that their predecesors and not on the fact that the predecessors are mediterranean. Option B modifies predecessors while option D focusses on mediterranenan and therfore option B is the answer.

James Joyce is Max's supposedly Irish ancestor
James Joyce is Max's supposed Irish ancestor.

The focus is on whether James Joyce is Max's ancestor. The first one focusses on James being Irish while the second sentence focus on James being the ancestor. Hence, 2nd one is correct.