I think what the author is talking about is using the MOSFET Rds as a current sense resistor. This is better than adding a current sense resistor. The Rds of the MOSFET is inherent (you are stuck with it) so you have to accept the losses it causes. But if you add a current sense resistor, you will incur even more losses.

The Rds of the MOSFET follows the simple rule of Ohm's law: V = I * R. If you know R and measure V you can calculate I easily. In practice, for control, you may not need to know R precisely. The control loop will still work because it adjusts the peak current target until the output is correct. Current limiting can only be as precise as Rds, however.

Directly sensing instantaneous inductor current by measuring voltage across the inductor is not really practical for obvious reasons. So the only practical way to sense instantaneous inductor current is indirectly by using a current sense resistor in series with the inductor. As previously noted, this adds losses, so it is better to avoid doing it when possible.

In a boost converter there is generally an N-channel MOSFET with source grounded. This makes for easy low-side current sensing. A buck converter will usually have P or N MOSFET on the high side which can be used for high-side current sensing. While it is more difficult to design the high-side sense circuitry, it is still very doable. Many IC's incorporate this feature.

I think what the author is talking about is using the MOSFET Rds as a current sense resistor. This is better than adding a current sense resistor. The Rds of the MOSFET is inherent (you are stuck with it) so you have to accept the losses it causes. But if you add a current sense resistor, you will incur even more losses.

The Rds of the MOSFET follows the simple rule of Ohm's law: V = I * R. If you know R and measure V you can calculate I easily. In practice, for control, you may not need to know R precisely. The peak current control discussed by this article will adjust the peak current point up or down as needed until the output voltage is correct. So the output regulation will still be good, even if Rds varies a bit from unit to unit, or with temperature. This is just the nature of feedback control.

Some controllers also have cycle-by-cycle current limiting. The article alluded to this, calling it over-current protection. This is not part of the control, per-se, it is just a feature that can help avoid inductor saturation and limit short-circuit current. This feature DOES depend on knowing Rds accurately. Basically, this is an absolute upper limit on peak current. If this current is reached, the controller will instantly turn off the high-side switch, regardless of whether the output voltage is in regulation or not. So if Rds varies a bit from unit to unit, then the absolute upper limit will also vary a bit, and the over-current protection will kick in at slightly different levels from unit to unit.

Directly sensing instantaneous inductor current by measuring voltage across the inductor is not really practical. For the peak current control method, the instantaneous current must be sensed when the high side switch is closed and the inductor current is ramping upward. However, in this condition, the inductor voltage is just Vin - Vds - Vout. Vds is small, and Vin and Vout are pretty much fixed. So sensing a nearly fixed voltage will not help you figure out the current. John D mentioned that it is possible to use a network to cancel the reactance of the inductor. When you do this, the inductor resistance becomes your sense resistor. I don't know much about that, but it seems like such a network would be a filter, and would thus affect the signal bandwidth making instantaneous current sensing difficult or impossible. It would work for average current sensing, though. 

So the only practical way to sense instantaneous inductor current is indirectly by using a current sense resistor in series with the inductor. As previously noted, this adds losses, so it is better to avoid doing it when possible.

In a boost converter there is generally an N-channel MOSFET with source grounded. This makes for easy low-side current sensing. A buck converter will usually have P or N MOSFET on the high side which can be used for high-side current sensing. While it is more difficult to design the high-side sense circuitry, it is still very doable. Many IC's incorporate this feature.