I'm betting the Linux kernel actually tests their config combinations. The complaint is that Chromium doesn't test turning off their allegedly-optional features, so when one of those features turns out not to be as optional as it claims to be, it's Gentoo maintainers who bear the brunt of discovering that (or users end up finding it out if it's not tested).

I can't tell you what Gentoo does with the Linux kernel in terms of USE flags; it's been over a decade since I last tried Gentoo and I don't want to spend the time looking it up since I doubt anyone truly cares. (If someone does care enough and looks it up, I care just enough to be curious to know what you find).