The active chemical in Preen is trifluralin
googling for 'trifluralin herbicide label' found this
It has recommended usages for many legumes and grain crops (not terribly relevant), but also tomatoes and orchards.
"spray between 4 weeks and just before sowing takes place"
Orchards and Vinyards
"apply to established crops in spring after weeds ... have been ploughed into ground"
Nothing mentions interval before harvest - though obviously there would be a few months between sowing tomatoes and harvesting them. Other chemical sheets I've read have been very clear on the acceptable interval between application and harvest.
My non-legal non-scientific opinion is that the preen company is just covering their butts over something not 100% already proven safe, but you could probably even eat from this patch by the end of the summer (when you would be harvesting fruit or tomatoes), but would certainly be fine next year.