So we've spent years shouting out the mantra "test for features, not browsers!" and now Mozilla, of all people, tell us that that was basically a bit impractical, and we should just go back to user-agent sniffing like we did 8-10 years ago?
And, you can choose to do feature detection if you wish. Here's a quick example:
https://cdn.polyfill.io/v1/polyfill.js?features=Array.protot...
More information about that can be found here: https://cdn.polyfill.io/v1/docs/api
No. What Mozilla did was explain their reasons for doing so. So at the very least your comment should engage with their stated reasons in some detail. Otherwise it's hard for the rest of us to know whether you've got a valid objection or you just replied without really reading the original article.