That sounds reasonable, but why would he think it would have the upper hand with regards to geometry processing? The data move engines and ESRAM wouldn't really help with that, would it, certainly not while Orbis is packing more CUs.
Unless there are as yet unrevealed customisations to Durango's GPU that give it a vertex/tri advantage.
Your guess is as good as mine, I suspect thuway knows more about Orbis than he does Durango and he's passing on information from another poster who also posted geometry speculation.