Yes the disk brakes are basically what people go by to distinguish early 2001s from later ones. Some call 2001. 5s the ones that came out when the 2001s would have normally hit the showroom floors. I think it was in July or Aug 2000 the rear disks became available so it was pretty close to the normal 2001 model year rollout.
MY 2001 trucks were built from Jan 2000 to Jul or August 2001.