According to the Books, not to the Stones

In many publications and internet pages we read that the Baptistery is the result of various transformations carried out since the early Middle Ages. But if one goes inside the attics cavities to look at the walls, vaults and masonry, which can be seen without coatings, there appears instead the perfect unity and homogeneity of the construction.
There is no trace of modification.

So, who must we agree with, the talks or the stones?