They’re not working on anything, this just means the WebKit previews or the odd product which embeds WebKit rather than use webviews can run regardless of OS support.
Apple ain’t going to enable features they don’t support on safari.
Apple ain’t going to enable features they don’t support on safari.
That's why there are regular "Webkit features in Safari X": that something is added to webkit does not guarantee it'll land in Safari, ever.
Hell, it took 18 months and 2 iOS/macOS releases for the AVIF support to go from landing in Webkit to landing in Safari. And that was something Apple wanted.