Their example of asn.1 also needs a schema.

No, ASN.1 has separate boolean, integer, and enumeration types, as well as separate "printable string" and "octet sequence" types.

Yes, which makes it a schema

OK, then what term would you like me to use to mean "the thing protobuf uses to define messages, commonly called a schema, that lives outside the encoded data itself"? The point is that ASN.1 can do certain things without it which protobuf cannot.