The most recent version of WebKit does not automatically mean that it's a scheduled build directly based off of their development branch. I agree that's the most reasonable way to do it, and it's what it implies. And it's easy to assume that's what they're doing. But they could just as easily be putting this in an intermediate tech preview branch and pulling individual commits.

The phrase "most recent version" could mean pulling from a dev branch, using the last successful build, pulling from a release branch, pulling from a testing branch, assigning arbitrary numbers and tags to commits and pulling from there, or even working from a staging repo where they cherrypick commits. These are all separate sources that could hve their own version series, and "most recent version" is a very relative statement. Anyone who's seen the divide between development and sales knows that phrase has enough wiggle room to fit a Challenger disaster inside, and marketing is Apple's specialty.

I really hope they start to pull the changes from WebKit. Safari sorely needs it. But Apple's not the kind of company you just take at their word unless it's independently verified. I get it if you want to believe. That's sweet. But I'd rather wait for the tests.

Even then, monthly builds are still not a public release schedule for Safari. It was six months with only minor fixes between 9.0 and 9.1. We're far more interested in a public stable release with usable ES6 support than builds with unstable features that won't be usable on sites this year. If preview builds were what would solve the problem, then the WebKit Nightlies would have been enough.