The comparison he makes is with pre-Buddhist stupas...
Ha! you beat me to it widford :-) When I heard about turf being brought from afar and laid on the primary mound at Silbury the first thing I thought was stupas. This from Wikipedia* on stupas, "The stupa is the latest Buddhist religious monument and was originally only a simple mound made up of mud or clay... They evolved into large hemispherical mounds with features such as the torana (gateway), the vedica (fence-like enclosure evolved from the vedic villages), the harmika (a square platform with railings on top of the stupa), chattrayashti (the parasol or canopy) and a circumambulatory around the stupa." Sounds a bit familiar ;-)Hear tell by the way that 3rd Stone is coming back next year under a different name.