This just hasn't been updated since it was forked from the initial OSS release from Google, I've started a discussion on the firmware Discord to update it.
The HRM aspect is mostly a small binary blob that ought to be fairly straightforward to re-integrate. The far larger issue is the lack of a Bluetooth stack.
ChromeOS originally was built upon Ubuntu, but Gentoo made more sense when starting to build for a rather diverse set of hardware, various flavours of ARM etc.
I stayed at Stonehenge for about 3 months last summer, great place. First hacker house I'd ever lived at (and I'll admit I wasn't sure about the idea at first), but it turned out to be a lot of fun.