Yes, that is why it is a deal breaker. I'll choose to run a different OS. I didn't say that GrapheneOS must support root. Just that I won't run it if they don't.
What should that support look like? Maybe have a userdebug build already built and available? I don't include a root account on hardened container images for some of the same reasons they cite. So including it for everyone and creating a way to activate it is suboptimal for people who don't want that trade off. A parallel build pipeline seems the most reasonable to me?
And I'm fine with you wanting root on the device you own. But you were implying that not having root means that you don't own your device. I disagree with that. You can totally own your device and not be root.
I think it is important, because I read a lot of comments that imply that "owning their device" means "owning the developers". And that's a wrong fight.
The real fight is that it should be illegal to prevent me from installing my preferred OS on a general-purpose computer.