It's interesting that this is generally not frowned upon from the hacker community, even though only supporting Webkit is clearly the same fragmentation issue we've all been upset about for so many years.
I imagine if it was "Sorry, we only support IE9" you'd get a different reaction.
The reason that there's so much focus on web standards is because Microsoft used their browser to force platform lock-in.
I don't see going webkit only as being a problem: Chrome is available on the three major PC operating systems (Windows, MacOS, Linux), it's free and it's not owned by a PC OS vendor.
Now if it was Safari-only or IE-only, then that would be a problem.
The Chrome trademark is owned by Google. Webkit is BSD and LGPL [1], and Chromium is GPLv2 [2] and virtually identical to Chrome except for branding and some proprietary plugins Google doesn't control the licenses for [3].
So yeah, while you're technically correct by your wording alone, I'd argue you're wrong in spirit.
Actually, Chrome is not the same on all three systems. I can't remember the site, but a while ago someone wanted to share with me a site using advanced CSS3 transitions and it didn't work as expected on my Chrome on Linux (the latest beta version at the time). Only after switching to Windows and using Chrome there (the latest beta version too) I was able to see it.
I would say that we don't mind because we understand that there are many features on "modern browsers" that are not available on IE 9 and below, but yet are web standards (ok, not all the time). Hackers also tend not to use IE, so that also helps explain it.
It really only sucks for general consumer-facing apps to not support a large variety of browsers. Otherwise, for specific apps with more tech-savvy users, just supporting a couple of modern browsers is just fine IMO.
The problem is that this is the same argument about why we had the "only supports ie6" way back in the day (and ie5.5 before that).
While I don't think that webkit will stagnate the way ie did, it's really amusing to see the webdev community repeating the exact same arguments supporting webkit-only sites that we were about ie5-only sites 12 years ago.
I imagine if it was "Sorry, we only support IE9" you'd get a different reaction.