The peak threshold is low so that losses from the current sense resistor are minimized. This is also why those dividers/diodes are used on the error amplifier output (to provide a 0-0.25V control signal from an amplifier with a much higher supply voltage).
In other controllers (mainly from LT) an amplifier is used on the CS pin to boost its voltage to match the operating range of the error amplifier.
If you really direct access to that many internal nodes, then you might as well roll a discrete controller. Or maybe use an external comparator which drives the CS pin of the control chip. That way you could still make use of the oscillator, latch, and driver in the control chip.