Earlier quoted context omitted.
Serious question: What are the "valid concerns" about people securing their computing devices against third parties?
This (I think) refers not to the people securing their devices against third parties but the vendors "securing" the devices against loss of profits. Essentially, the question referenced here is that of ownership. Is it your device, or did you rent it from Apple/Samsung/etc. If it is locked down so that you can't do anything you want with it, then you might not actually be its owner. ___ _Ideally_ you wouldn't need to…
Both goals actually are possible to implement at the same time: Secure/Verified Boot together with actually audited, preferably open-source, as-small-as-possible code in the boot and crypto chain, for the user, the ability to unlock the bootloader in the EFI firmware and for those concerned about supply chain integrity, a debug port muxed directly (!) to the TPM so it can be queried for its set of whitelisted public keys.