My guess is that "cuban seed" means seed that comes from cuba (doh)
When Castro took over, a bunch of cigar makers fled the country with tobacco seeds in their pockets and set up shop elsewhere. These are the cigars we smoke today.
Yeah the seeds come from cuba, but that really means nothing. The conditions of the soil and climate matter much more than the specific tobacco plant. That’s why cuba can make a good cigar using only cuban tobacco, the micro climates of that tiny country are enough to make tobacco with distinct flavors.

So in short, "cuban seed" means absolutely nothing.
Someone correct me if I'm wrong.