But the WHS is only about the prehistoric caves in the region.
No - not strictly I think - the Zapotec ruins at Yagul ARE within the boundary -so, for instance, its ball game court means that Connection is valid whilst it has no mosaics (or at least to my knowledge no significant ones). On the other hand we could say that these Yagul Zapotec remains play absolutely NO part in the OUV of the site. In which case it is a bit surprising that they were not excluded from the inscribed area! As I look at the map it does a slight NW "jink" to include those ruins and could easily have missed them out.
The AB evaluation does refer to the Yagul Zapotec site ("The remains built mainly of stone and mud mortar consist, principally, of the Palace of Six Patios or 'labyrinth', the classically designed Ball Court facing east-west; a U-shaped building on one of the highest
points of the site; the Council Room, built on a platform; five further patios and the Fortress following the natural, almost circular form of the highest hill.
It appears to give the Yagul Zapotec site relevance as follows"The site of Yagul reflects the break-up of Zapotec hegemony in the Oaxaca valley, with the abandonment of Monte Alban (inscribed in 1987), and the subsequent diffusion of power and development to smaller urban centres, such as Yagul. It is suggested that these states would not have reached such levels of socio-cultural sophistication had it not been for agriculture. Yagul thus represents a different stage in the development of the valley."