Alex Russell’s position has always been that web developers (especially library authors) need as many browser features as possible to help them build the most flexible / capable web applications possible. He has consistently pushed for browsers to ship new features ASAP, with the idea that developers can paper over the differences or work around a buggy API, but can’t turn nothing into something. My impression is that he joined Google because he thought that was the best way to further that pre-established goal; it’s not fair to say that he’s just promoting this because he thinks it is in Google/Microsoft/Chrome’s interest.
By contrast, Safari developers’ position has long been that new browser features should be carefully evaluated and then implemented at leisure, because this is a long game and we don’t need rush jobs. Their focus is more on client-side resource use (it’s shocking how far behind Chrome is on this one), correctness of the features they do decide to implement, performance, and user privacy (Google’s whole business model is ubiquitous surveillance).
These two positions are both defensible, but are at odds with each-other. For Safari developers, Chrome trying to ram big piles of half-baked new features down every browser’s throat ASAP is just a recipe for churn, and getting developers to adopt them right away so that users then come complain that “Safari is broken” is endlessly annoying. For Chrome developers (or web developers who want to build on new shiny features right away instead of waiting 2+ years for cross-browser adoption), Safari lagging behind is “holding back the web”.