It's very annoying that they restrict themselves to Pixels. I get they can't guarantee all the security features they want on other phones, but even a subset of those security features and the other advantages like the lack of cruft would make it very attractive to be able to run on other phones.
They are working with a partner to get their own phone hardware, hopefully by next year.
I can understand the frustration, but it wouldn't be right to say they 'restrict themselves to Pixels'. They believe strongly in a standard for privacy/security of people's personal devices, and unfortunately only Pixels are close to meeting those standards. It's not even like Pixels are their ideal device.
I feel the frustration should be targeted at OEMs that don't meet very reasonable requirements like minimum 5 years of monthly (timely) security updates.