Try this

WITH Capacities AS (

select en.NAME as ClusterName, ROUND(CAST(CPU_CORE_COUNT as FLOAT)*CPU_Hz*count(cast(hs.HOSTID as BIGINT))/800000000,0) as Hub_Capacity

from VPXV_HOSTS as hs WITH (NOLOCK,NOWAIT)

full join VPXV_ENTITY as en WITH (NOLOCK,NOWAIT) on hs.FARMID = en.id

where en.TYPE_ID = 3

group by en.Name, hs.CPU_CORE_COUNT, hs.CPU_Hz

)

SELECT SUM(Hub_Capacity) AS TotalCapacity

Pretty sure John just forgot to remove the sum in the cte.