No, it is not. It's webkit. Safari is based on webkit, but the webkit project at large does not decide what does and does not go in 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.