:squint:
One of those moments where I'm not sure if the article is glossing over what is commonly understood or doesn't get it. The level of danger a MosCA poses is the same whether you are its customer or not. The danger is that CAs are decentralized in a "anybody can issue anything" way. If you root-trust MosCA, they can issue certificates for anything. "I get my regular certificate from them" does not make that easier or harder, because that process doesn't expose the private key in sane setups. (Yes, I know most CAs have insane setups because customers can't keep two files around for two days and find them again.)
There used to be HPKP, where a site could say "I guarantee my certificates are issued by CA XYZ for the next n days", but that was dead before it got off the ground, because it's only "trust on first use" and requires things like backup keypairs.
And no, don't answer "what about CAA", this is not what CAA does. CAA is verified at issue-time by the CA, it's protecting against social engineering.