why can i activate the stall warning at cruise speed while still straight and level, or even in a dive, just by pulling sharply on the stick?
I'm really not sure if you're serious or not, but for your benefit: Regardless of the aircrafts speed or configuration, there is still 5-10kts between stall warning and aerodynamic stall. The usage of knots as a unit of measure is to offer something tangible to the person at the controls. To make the statement "the stall warning activates 2 degrees before the stall" is useless in practical applications.
Does this clear the previous statement up?
The fact that reference to speed above aerodynamic stall for pusher activation is mentioned in AFM's of slightly more sophisticated aircraft than a 172 would indicate that it's a reasonable datum measure.