In some cases feature detection is not enough because while the browser may define the method you need it’s implementation is broken. A good example of this was indexeddb was horribly broken in safari and even doing feature detection was tricky so you had to check the user agent to see that it probably had a bad indexeddb and work around it. It’s never ideal and there is always possibly and more clever work around but sometimes you have a tight timeline and need a site to work in a specific browser.

Note: I think safari has since fixed indexeddb?

Google is not a single entity. Chrome DevRel and Docs web front end are not even remotely related teams, and DevRel cannot dictate practices to other teams.