Im not sure he's talking about the standard zone 3 in the way people talk about Pogacars zone 2 training, but about the third system of energy generation in the body.
Above Zone 2 there are many more zones based on which model you use (traditionally between 5 and 8 zones in total), look for example this example from BikeRadar:
When talking about anaerobic efforts I think it's definitively talk about the higher end of Treshold zone and above Vanthourenhout is speeking about in this specific case. That also rhymes with Remco saying he need to improve his 3-5 minute effort to be able to stay with Pogacar during these longer seated attacks without killing himself.