It’s not incompatible with open source. It’s incompatible with free software. If Apple or Google or Microsoft or the government needs to “bless” your build, then you have no freedom to actually use your build.

Your "development build" is another person's daily driver.

Case in point: GrapheneOS (or any other custom Android distro) is unlikely to be able to ever pass remote attestation, even a signed, secure boot build with the bootloader relocked, because it's not the original OS for the hardware.

Same goes for any desktop Linux.