My rough understanding is that's essentially what happens. Mirror neurons (it's a relatively small area) are not, and could not be, a completely isolated replication of all the other subsystems in the brain. They just connect observation or imagination of what someone else might be experiencing to your own respective motor/cognitive areas. If there were no filter/activation subsystem for what someone else was experiencing, you'd be experiencing everyone else's experiences all the time. Or experiencing none of it.