That's not entirely true, if yields are preventing MS from launching with little actual product, that's a pretty big damm issue for them. Besides that, we don't know how the contract is setup for MS, it could be on a wafer basis at which point MS is liable for yields. Dave never actually confirmed anything, he just challenged the notion that it must be MS who is at risk for poor yields.
I thought Global Founderies did all the manufacturing now, not AMD? In any case it's likely somewhat split between the manufacturer and the client. Any issues (if they actually exist) affect both of them. MS would simply have the call on how to deal with the issues, especially if it was something related to their custom design. Bottom line, until there's some kind of official confirmation, as far as we know there are no problems.