MQTT doesn’t specify the payload format. You can easily have a topic with the data in protobuf and another with json.

The post seems more like an argument against protobuf in general than against its use in telemetry. The main complaints seem to be that the library is big and that nested serialization is tricky because of design decisions.

In regards to binary size, using whatever serializing you're already doing for your main data seems like a win for telemetry. Including telemetry with your main data has pluses and minuses, but using the same serialization is generally fine, unless your serialization can fail, in which case it's hard to report on how many times telemetry serialization fails, because you might not be able to serialize it.