The trouble is that the value for Rds-on depends on the Vgs voltage used and how much current is going through the drain. In the case of this MOSFET, 17mΩ is only valid when Vgs is 10 volts.
Unfortunately, the datasheet for this transistor doesn't include a Vgs vs. Rds-on graph. But it does provide a Id to Vds graph for different Vgs values.
The blue line represents a Vgs of 5 volts while the orange line is 25 amps through the drain. Notice a problem? The manufacturer is telling us the transistor won't be able to handle that much stall current. What will likely happen is that the transistor will effectively open the circuit, cutting current to the motor. If you want to keep 25 Amps running through the motor, you'll need to drive Vgs with a higher voltage.
For the moment. let's just say you stay with 5 volts. If the motors draw 10 amps of current, this graph shows the voltage drop across the transistor will be 300mV. Using Ohm's law, we can see in the linear region, the Rds-on for the MOSFET will be about 30mΩ. With a Vgs of 5 volts, that is a better Rds-on to calculate with.
With that, you can calculate the power dissipated by the transistor and the junction to case temperature, as shown in the video.