If you wish so you can gain root privileges on your own in your own build or with modifying GrapheneOS existing builds. It wont be compatible with GrapheneOS provided updates because of signature mismatch
That device, and the Debian derivative it runs, are not private or secure.
Freedom of computing on Librem 5 doesn't end with the root account. It also allows to natively run any desktop software and develop it in any language, without reliance on Google's decision on how one must use the phone, how your OS must evolve and when you may get your updates. Or install a completely different OS from different developers, because there is no reliance on anything proprietary at all.
How you can call a device with a ton of opaque binary blobs more private and secure without mentioning this fact is beyond me. I do not call Librem 5 more secure. But its security depends on what I choose to run on it. And I only run trusted software, so it can be secure.
The protection is achieved through security. The major goal of something like GrapheneOS is privacy, which needs solid security as a prerequisite.
The blobs, while proprietary, are not opaque. They are able to be examined and they are.
The security of a device should not be dependent on what you choose to run on it. You should trust and be able to verify that the platform on which you are running the software prevents something malicious from accessing data which doesn't belong to it or otherwise violates the rules set by the platform (OS).
In this respect, the Librem 5 would do a horrible job compared to even stock AOSP. Thinking that you are secure because you only run "trusted" software on an insecure platform is cope.