There's some hand-wavy magic in this article, which was written months after doing the actual work. One aspect of that is I made a lot of test data and while yes, it could be any number of things, dealing with Occam's razor meant as I saw what looked like a message type (and not just a byte sequence which represented many other potential protobufs) I went with that as the conclusion. That conclusion seemed to hold as I test many other, more complicated notes generated from different types of devices. Hence why I present it here as a message.
But, yes, you're absolutely right that this is not the protobuf definition used by Apple (which is copyrighted) and there is certainly information missing from it. My goal was simply to be able to recover what I see on the Apple device's screen.
Presumably its contents. The included protobuf-inspector output shows it's a valid submessage, including length-delimited fields having valid lengths. A two-deep string matches the expected title. The tag numbers are also in order, which adds plausibility. (Tags aren't required to be in a particular order but IIRC the stock implementations in most languages output them in ascending tag number order for known fields.)
This is pretty strong evidence. If it's not strong enough for you, reverse engineering might not be your thing. That's okay.
Nit: message is isomorphic with bytes, not string. string has the additional restriction of being valid UTF-8. I didn't check, but I wouldn't be surprised if field 2 is not a valid string.