GrapheneOS and AOSP are Linux-based and there are no closed source kernel modules. They aren't somehow not actual Linux due to not using systemd, glibc, binutils, GCC, pulseaudio/pipewire, polkit, NetworkManager, GNOME, etc. If that's what you mean, you should say so, because those userspace components are not Linux and not using those doesn't make it any less of a Linux distribution. Is Alpine not a real Linux distribution? Is it only a real Linux distribution if it looks like what you're familiar with? More developers are familiar with Android than the desktop Linux software stack. More work goes into it. Far more apps are written for it, and that includes a very active open source app ecosystem.

Sticking to an LTS kernel branch for the lifetime of the device isn't due to anything closed source. GrapheneOS only supports devices with proper security support for all the firmware, drivers, etc. and again there are no closed source kernel drivers. We can support pretty much any mobile device with alternate OS support since any serious one will have AOSP support. Most devices have lackluster security and don't meet our requirements. We're working with a hardware vendor to get a non-Pixel phone actually meeting reasonable security requirements.

Librem 5 has a bunch of components where they are not shipping updates. You have things very much backwards on that front. The Librem 5 does not come close to meeting the security requirements to run GrapheneOS. It has a bunch of poorly secured and insecurely configured legacy hardware often without proper updates available, components that are not properly isolated via IOMMU, no secure element or all the stuff that comes along with that (HSM keystore with a nice API used by apps, Weaver to make disk encryption work for users without a high entropy passphrase like 7 diceware words, insider attack resistance, working attestation not depending on hard-wiring hashes and a lot more) and many other things. The OS they use has a near total lack of any systemic overall privacy/security work or privacy/security model and only falls further and further behind. The most exciting feature for securing devices right now is hardware memory tagging support in ARMv9, but there are years and years of tons of important privacy/security work done in a systemic way across hardware/firmware/software which are missing there before worrying about stuff like that.

Marketing something as private/secure and spreading tons of misinformation and outright lies about the mainstream options doesn't make it secure or more secure than those. It's actually pretty funny that they mislead people about the isolation of hardware components like the cellular baseband in other devices when the vast majority of mainstream phones (iPhone, Pixel, Qualcomm SoC devices, Exynos SoC devices) have it done quite well when they don't. Strange that they get away with these games of misrepresenting things, hiding the fact that they still have entirely proprietary hardware and near entirely proprietary firmware for the SoC and other hardware components, etc. Hiding proprietary stuff doesn't make it go away. Not updating it doesn't make it go away and simply ensures a highly insecure device.