When you see or hear the down shifting the driver is not using engine compression to slow the car he is rev matching (getting the car in the proper gear for the turn and exit).
You do use the engine to slow the car/bike/vehicle down. You use everything in your arsenal to slow the car/bike to make the corner, including engine compression.
The reason you "Rev Match" is to avoid wheel hop which is worse than not down shifting at all.
If you aren't on the gas or maintaining enough HP output from the engine to carry current MPH, you're slowing down with/using compression.
I do agree it's more important to be on the gas earlier exiting than to bomb it into the corner.
