But regardless, this entire article is about how to have an actually secure boot on Linux (and not remote attestation), something which is certainly good for the user. Otherwise you're actually more easily susceptible to malicious actors stealing your data on the system you "trust". None of the steps listed here require trusting anyone other than the TPM (which is admittedly a flaw so long as we cannot audit them) and you can even use your own keys in pretty much all cases (provided -as discussed in other comments- MS hasn't f'd that up). I personally use a boot-stub based booting method with my own SB keys (but ironically, don't encrypt my root, so go figure), so I can vouch for the fact that it was actually quite painless to setup. Please don't jump down the slippery slope before you actually try these methods and realize that this is just as easy to deploy (you only need to disable secure boot first) and certainly a more secure option than using shim and an unsecured initrd.
Also, I don't understand where remote attestation entered the conversation here, and I also don't see why that can't be a community based thing (al la let's encrypt is now everyone's CA) where you can choose your providers or even roll it yourself.