Germination is the process whereby a seed comes to life and begins to grow. Sprouting is what a germinated seed does when it pokes above the soil surface.

Fantastic! I was hoping Preen would kill buried seeds plus weeds that have poked above the soil in the 6 days since I weeded my garden.

"prevents new weeds from sprouting" is not exactly the same as killing weeds that have poked above the soil......those have already sprouted and depending on how far they have developed, the Preen may have little to no impact on them.

I realize that but Preen claims to do BOTH.

Actually, it doesn't :-) It may be a somewhat subtle distinction but "preventing seeds from sprouting" is quite a bit different than killing seeds which have already sprouted and now appear above the soil surface. There is no prevention involved in that instance and the effectiveness of Preen may be limited.

Let's be accurate in this reporting:
"EPA considers trifluralin to be a possible human carcinogen (11). This
classification is used when there is limited or uncertain information
indicating that a chemical may cause cancer in animals receiving high doses of
the chemical over the course of their lifetimes. In a 2-year study of rats
fed 325 mg/kg/day, the highest dose tested, malignant tumors developed in the
kidneys, bladder and thyroid (7, 12). Because there is a possible increase in
the risk of cancer to humans, EPA's Lifetime Health Advisory level for
trifluralin drinking water includes an additional safety margin "

Since humans generally don't eat Preen, and are even less likely to do so in high doses, the chances of this chemical actually causing cancer are rather low. And ALL kinds of substances - not just pesticides - can cause cancer or be overly toxic if ingested or exposed to in high enough doses.