Well, it indicates his chin was not glass, as he undoubtedly took some hard shots over the course of 64 fights, but given Johnson's athleticism and defensive skill, along with his proneness to knockout losses before before developing them and after they declined, I believe it resulted primarily from those factors rather than from durability.
No, that's just an assumption from looking at numbers. We've seen that question answered in the most unequivocal and direct way possible.