No, you have it backwards. The slack cable tells the transmission that the rpms are being developed without throttle movement so it shifts since there is no load.
The idea is to put the trans in the appropriate gear for the load being delivered. If ones throttle is heavy then the trans stays in the lower gear longer to gain torque advantage and develop horsepower.
PS if you have a slack cable and are shifting late, the cable is probably frayed and stuck out. This will definitely cause late shifts.
Bosch Master, ASE Master, L1
33 years MB technician