To the best of my knowledge, Doug Heath is no longer employed with Seminis. They had a big shakeup about 2 years ago. I know this because I am in contact with one of the people he worked with.
Re what defines an F1 hybrid, it is better to look at it from the reverse position by asking how consanguinous the genes are in a given plant.
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Consanguinity - kinship characterized by the sharing of common ancestors
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The more closely related two parents are, the more genes are identical. In a typical open pollinated tomato, the genes approach 100% identical but never quite reach it because random mutations occur at a small but significant rate. When a tomato plant is in this condition, the seed produced by a plant is almost a clone of the parent plant. The level of vigor and production and disease tolerance is entirely decided by whatever genes are present with little or no possibility of heterosis.
The polar opposite would be two plants that are so far differentiated that genetic barriers prevent reproduction. This is the case between tomato and potato for example in that they share 98% of their genes, but the differences are so great that a tomato cannot successfully reproduce in a cross.
In between these extremes are plants that can successfully cross and reproduce though various levels of difficulty can occur. Within the tomato family, S. Habrochaites is a very interesting plant. It is more cold tolerant, more stress tolerant, more disease tolerant, and more adaptable than cultivated tomato. Unfortunately, S. Habrochaites makes nasty smelling and tasting small greenish/white fruit on a plant that stinks. What if you could bring some of those good traits over into a commercial tomato? Well, this can be done, but with the condition that the domestic tomato must be the female parent in the cross. Collect pollen from S. Habrochaites, dust it on the stigma of S. Lycopersicum and the resulting seed will be an F1 hybrid. The level of genetic difference is very high and though they share common ancestors, they are at least so different that very high levels of heterosis can occur. The plants grown from the seed in this cross are useless for producing edible tomatoes, but they can make outstandingly good rootstocks for grafting tomatoes.
What about making F1 hybrids between two S. Lycopersicum varieties? They are in the same species and generally will be 100% cross compatible. The problem is that the domestic tomato has an extremely shallow genetic base. The number of plants that were domesticated from S. Pimpinellifolium contained only a small amount of the genetic variation in the parent species. This limits the amount of heterosis that can be expressed in any cross. So if I cross a heart shaped variety like Anna Russian ("el" gene) with a beefsteak ("f" fasciated gene) it will produce fruit that are fat hearts such as Tastiheart.
So what level of genetic difference makes an F1? A better way to ask is how much heterosis - the increase in growth, size, fecundity, function, yield, or other characters in hybrids over those of the parents - will be produced by a cross. While there are no absolute rules, the more different the two parents, the more likely their offspring will exhibit heterosis and the more alike two parents, the less likely their offspring will be exceptional.
Now it is easy to see why with tomatoes you can do all kinds of funky things with shape and size and color, but it is very difficult to get a plant to express heterosis. When crosses are made to wild species, heterosis can be extremely intense. This is why Maxifort rootstock is a cross of one wild species variety and one domestic tomato variety. Heterosis turns the resulting plant into a powerhouse as a rootstock.