Yeah- this is the only reason there's any openness at all in the mobile space right now. Android forks couldn't exist at all - much less postmarketOS - if the underlying hardware and drivers weren't open and available.
A lot of bootloading / unlocking has been figured out by reverse engineering closed systems. It turns out Google is one of the few ones providing easy tools to do this on nexus / pixel phones, even though they didn't have to.
They still provide the binary drivers/firmware from upstream because there's no replacement for those.