I can assure you that it is not true.
Sperm begins as "germ cells" in the "testes", they then move up into the "epididymis" to mature for a surprisingly long time (several weeks). They eventually move along the "vas deferens" (tubes from testes) and are mixed with fluids from the "prostate" and "seminal vesicles" to form what is commonly known as "semen".
This whole process can take upto *seven* weeks!
Pre-cum is fluids from the "prostate" and "seminal vesicles" which are situated along the "vas deferens".
The reason you can not be 100% there is not any sperm in pre-cum is because you can not guarantee that the are no spermatazoa (single sperm cells) in the "vas deferens" at any given time.
Even if a guy has already ejaculated, not ejaculated for over 24 hours, has just peed, anything. Nothing can ensure (not even a vasectomy can be 100% [although it is usually fairly accurate]) that no sperm are included in the pre-cum
it only takes *one* sperm to fertilize an ovum (egg)