How many laptop users frequently change their bootloaders? Is this a thing people do all the time, or just the first time transitioning from Windows to not-just-Windows?
Are we looking for problems to solve? If a bootloader can bootstrap your OS kernel, that's all you need, no? So what are we talking about here? If you can throw Grub on there, and Grub gets you a kernel / rootfs, we're good, right?
Nvidia already knows how to make a proper EFI setup that doesn't lock anyone out, and has the requisite EFI drivers for enrolling keys in the TPM to approve any OS you want to load through Secure Boot.
Nvidia is working with Microsoft to make sure that the arm64 variant of Windows works on the thing.
Unless you have some concrete proof that this barely-announced device will be irretrievably locked down to Windows-only booting, what are we complaining about?