SealTite seems to me to be a lot stiffer and closer to the consistency of TexTite than BCY ML16 or White Lightning, so that's probably why they recommend the old school method of rubbing it in.

I like having the wax burnished into my string to keep moisture and dust out from between the fibers, so I still rub the wax down good with a piece of soft leather. It also helps to warm up a new string/cables like that to help them settle into their working lengths quicker, IMO.