Yes I'm pretty sure this is what they do. The DHCP from the router gives 75.75.75.75 and 75.75.76.76. I've tried overriding that with different resolvers in my /etc/resolv.conf and it doesn't work. And logging in to the modem/router config does not offer any option to change DNS settings.
I just tried it. I enabled it at the "Max Protection" level, used the default provider setting (Cloudflare) and it works. So it seems the answer is yes. So that's a pretty simple workaround that covers most cases. I'm guessing that most of the DNS lookups that people would want to be private are happening via a web browser.
