In fact what this is counting, is a message received from a specific ICAO address AFTER it has been determined that its position is valid.
The data that composes an aircraft position, height, track, speed, squawk etc, is actually split over a number of different packets of data, and it is only the collection/union of packets (tagged by the ICAO code) which gives an overall picture.
So once the overall 'picture' is assembled, that is when we start counting.
Until that point, the data is incomplete, and therefore in effect a bearingless target.
I think you are kind of right in what you are saying, that there are many types of transmission on 1090, but here we are specifically counting packets of data which will contribute to a mapped aircraft appearing on your navigation display.
Thx
Lee