That means shit, if someone can compromise your bootloader in an hotel or some other public place then they’ll get to your data either way once you turn on the phone. This is one very small and very important detail that all those tech youtubers pro-privacy, security and whatnot love to ignore as it is the really hard one that makes all the difference.
Secure boot is a complex subject and it requires a lot of work and checks to make sure nobody tempered with your device and Graphene / Pixel are the ones that really give a shit about that (except for Apple that wants to block jailbreaking and pirated Chinese app stores at all costs).