I think it's not as good as you think. There is a difference between "it happens to be open" and "manufacturer cares", and here it's more like the former. The experience here is in fact identical to e.g. using any device whose bootloader has been cracked. You are locked to very old kernel versions, or attempts to run a mainline kernel in various degrees of stability (or lack of). You cannot really modify the existing user interface to your liking, as it is even more closed than Android; it's either take it or replace it entirely. And if you stay on the official firmware then there is a non-negligible risk that they will lock it down. I've been through that route...

It's in no way comparable to a some other devices (mentioned on HN too) where e.g. manufacturer actually cares about it. I agree it's technically better than "locked down bootloader with few chances of ever being unlocked" like some newer devices are, just not "miles better".

Plus the fact that it does run GNU/Linux rather than Android makes it more hacker-friendly out of box, at least for some types of hackers.

Yes, I meant "open" in the sense that you do not need to jailbreak it to run software that has not been approved by the manufacturer, not "open" in the sense that the operating system is open source.

Although, I think a lot of their operating system is open source and on their github page (linux kernel and uboot configuration): https://github.com/reMarkable

Their Qt-based shell software, xochitl, is not open source.