Opinions are my own.
But if you change a schema, be it DB, protobuf, whatever, this is the major thing your tests should be covering.
This is why people are so amazed by it.
The document also doesn't say there wasn't testing in staging or prod.
So do we. If you change the schema it doesn't matter if it's unit testing, you either have a validation stage for all data coming in beforehand to check these assumptions (the way many python people use pydantic on incoming data, for example), and that must be updated for every change, or every unit must be defensive against this sort of thing.