As shown in the original circuit, the voltage at the non-inv input could be anywhere from ~1V to about ~11V, depending on where the wiper of P1 is.
If you left out R2,R3, the entire 12V would appear end-to-end across the 10K pot. The non-inv input could then be adjusted from 0V to 12V as the wiper is moved.
The discussion about current limiting into the non-inv input of the 741 has to do with what happens as the wiper is run all the way up, and the input goes to the same voltage as the 741's V+ pin?