I ported their Hubris kernel to RISC-V and ARMv6 before applying for a role to work on it, so thought I had reasonably strong materials, but got rejected.
I fully understand and empathise with their reasons for not providing feedback, but it does mean you're left not knowing whether you were a hair's breadth away from getting in but were behind a better candidate and it would be worth trying again in the future, or rejected for other reasons that no improvement in materials will make up for. As a fully signed-up member of imposter syndrome club I'll obviously lean towards the latter ;)