Not true. For example Libreboot currently supports 2 ARM laptops. The way I understand it is that Libreboot uses U-boot as an extra bootloader, kinda like you would run GRUB after UEFI. U-boot can also just work on it’s own and Coreboot ARM devices are rather the exception.
I’d argue chain loading coreboot/libreboot from u-boot isn’t really “supporting it” as much as it’s just extending it, but fair enough. In the end it’s still using u-boot with extra steps.