If you look at the IRF7201 datasheet again, you'll notice the Rds(on) at 10V == 30mΩ, and 4.5V == 50mΩ.
Being "fully on" just means that you have hit the top(or bottom) side of the Rds(on) curve. That the resistance is about as low as it's going to go, regardless of how much more voltage you apply to the gate. There is no real set point at which this happens, and more of an acceptable point to the application at hand.
The DS shows the gate threshold voltage to be only 1V. That's when the MOSFET starts to turn on and pass current, but at a very high resistance. It's not until the gate hits ~4V that the resistance finally starts hitting an acceptable range.
Figure 6 of the DS is showing that using only 4.5V at the gate will hit a maximum saturation of the drain-source junction due to the field not being large enough for the entire channel to conduct. It's not "fully on" yet for that amount of current. If you wanted to pass more current, you would have to drive it with a higher gate voltage.