The web flourished due to its lightweight, comparatively easy-to-implement protocols. Applications could be built cheaply and easily on top of it. VCs and many entrepreneurs profited handsomely. Much of the past 20 years in tech has been pushing this concept to literally every person on earth.
Compare to the 'fat protocol': The protocols are tough to write, and tough to implement. The value is captured at the protocol level, not the app level, so there's little incentive to create inventive apps.
Few apps will be created on such systems, so few successful consumer applications will created with them. Seems like the winner here is in business-facing systems. For USV, it strikes me as peculiar to favor the 'fat protocol' and back other types of businesses (e.g. consumer).