Even then that's only for the boot chain after the application boot loader - you for example can't run custom UEFI apps in the Qualcomm eXtensible Boot Loader (let alone custom TrustZone applets or Hexagon DSP firmware) as those are still locked down to Google's RKH, and similarly Google also (unlike a number of Chinese vendors) does not offer their-signed firehose emergency recovery blobs in case of a hard user brick (say, erasing the GPT on the flash).