Timing capacitor and resistors -Reg
The datasheet says that


  […] the amplifier output voltage is then compared to the linear voltage ramp at CT.


"Linear" implies that the capacitor is charged with a constant current. So RT is not simply connected in series, but is used to program a constant current source.

We do not know how that chip implements this current source. But if we assume a current source like this:

(source: Wikipedia)

Then we could make it programmable by replacing the Zener diode with the RT resistor and letting a constant current flow through it; the varying resistance will then result in a varying reference voltage.