I can't parse what the rep is saying by applying logic.
"Mantle has no obvious benefits, we don't know much about Mantle, DX12 being a lower level API has obvious benefits. "
I think this guy is full of crap claiming not to know much about Mantle. If he actually doesn't know much of anything about it, then don't make yourself look stupid by saying it offers little benefit. He knows exactly what it's all about but to save corporate face won't say anything postiive but will do so when talking about DX12, which he knows full well seeks to do what Mantle does.