I found this a bit confusing too. I think you could have two things if they have /e/ OS already: they’re either Murena, like mine (in which case /e/ foundation has already unlocked the bootloader), or they’re true stock (like you would get from Teracube), where someone else has installed /e/ OS aftermarket (but would need to unlock the bootloader first). Hypothetically, you shouldn’t be able to tell the difference, and it shouldn’t make much difference for rooting, but I did have to root my Murena slightly differently from the method discussed here.
One way to get an idea of which kind of installation you have is to boot to the recovery software. If you go to “Developer options” and enable “Advanced restart,” you should be able to reboot directly to recovery by holding down the start button and selecting restart then recovery.
If the recovery is /e/ foundation software, then you probably have a Murena, if it’s TWRP, then someone installed /e/ OS on a stock phone. Either way is fine, it’s actually a little easier with TWRP, in my opinion.