That's not true. It does have root support built in with 'adb root' that then runs future shell commands as root.

The factory image has this disabled, though, so you do need to flash a build that supports it. That would be a 'userdebug' build variant ( https://source.android.com/setup/build/building#choose-a-tar... )

So strictly speaking you can have root through purely first-party, open source, AOSP tools exclusively. It does, however, require doing a full build & flash yourself. That process is at least documented, though, so there's that. Just think of it like Gentoo