This sounds like web.dev are advocating devs don't do feature detection and progressive enhancement, and adopt a "don't use a feature until it's in all browsers" mindset. I mean, you should detect support regardless just in case, but if you approach building apps with a more "I'll use whatever the user has available" way of thinking you can deliver better experiences (in my opinion), even if it makes a dev's life a little harder in some cases.
It's a surprising move from the web.dev team to say the least. If I was being cynical, and I am, I'd suggest this move is designed less to be 'helpful' and more to highlight how far behind Chrome other browsers are. It's like lots of pages are saying "This is a lovely feature, but don't use it because of clunky old Firefox" rather than saying "This is a lovely feature, and here's how to use it, including a polyfill/workaround for browsers that don't support it yet."
It's a surprising move from the web.dev team to say the least. If I was being cynical, and I am, I'd suggest this move is designed less to be 'helpful' and more to highlight how far behind Chrome other browsers are. It's like lots of pages are saying "This is a lovely feature, but don't use it because of clunky old Firefox" rather than saying "This is a lovely feature, and here's how to use it, including a polyfill/workaround for browsers that don't support it yet."