From the last interview question in the article (pertaining to Arm):

> We don’t really try to steer the market one direction or another; we just want to make sure that good options are always supported.

Sounds like their priority is to support Steam on the hardware consumers are currently using. Given that, it makes sense they'd go Arm in the Steam Frame, because Fex alone is already a massive undertaking, and Snapdragon is a leading mobile chipset for performance and power efficiency.