Apple is the manufacturer that is honoring the manufacturer's warranty (and/or additional warranty options). Google is not the manufacturer of the Nexus 5X/6P, that's Huawei. So if you didn't buy from them, they send you to the manufacturer, like most retailers would after the initial return window, even if you did buy it from them.

I agree that it's bad for Google's image to operate that way, stick their name on a phone and then shrug their shoulders and tell you to talk to the real manufacturer. But that distinction is why Apple supports most iPhones no questions asked, and Google wants to know that you bought it from them. They're a glorified retailer licensing out their brand to Huawei.