Per comments from the article earlier today/yesterday, it was because a feature wasn't supported in safari/mac, and didn't have the regression in the fallback methods for caching, so they let it be for non-windows. They've since updated the method, and the next release should have it in place.
Also, not everything is detectable via feature detection, there are bugs specific to browser classes/oses/versions that require useragent parsing/sniffing to target.
Also, not everything is detectable via feature detection, there are bugs specific to browser classes/oses/versions that require useragent parsing/sniffing to target.