If we are being picky about vocabulary, aren't heirlooms also hybrids? At some point in the past, they were selectively bred by people.

That is not to say that all hybrids can be legitimately called heirlooms - just that the two categories aren't mutually exclusive.

a sibilant intake of breath

Laura gets it right, Sindark.

From the Brooklyn Botanical Garden website:

On F1 hybrids (like Sweet 100s and Sungolds):

The term "F1 hybrid" means the first filial generation made by crossing two different parent varieties, the offspring of which produce a new, uniform seed variety with specific characteristics from both parents. For example, breeders may choose to cross two tomato varieties to make an F1 hybrid that exhibits the early maturity of one parent and a specific disease resistance of the other. The unique characteristics of an F1 hybrid are very uniform only in the first generation of seed, so seed saved from F1 plants will not come true if replanted and may exhibit many distinct types in the second generation, often reverting to various ancestral forms.

On open-pollinated seeds (like green zebras):

Open-pollinated seeds are a result of either natural or human selection for specific traits which are then reselected in every crop. The seed is kept true to type through selection and isolation; the flowers of open-pollinated or O.P. seed varieties are pollinated by bees or wind. Their traits are relatively fixed within a range of variability. For example, if I grew the 'Brandywine' variety of open-pollinated tomato in dry northern California summers year after year and saved seeds only from the best-tasting, earliest- ripening fruits in my climate zone, I would have a locally adapted strain of 'Brandywine', different from the 'Brandywine' grown by a gardener in humid, rainy Alabama who has been saving seeds from fruits that produce very well in his or her climate, rather than my California conditions.

Finally, on heirlooms, a subset of open-pollinated varieties:

All heirloom varieties are open-pollinated, but not all open-pollinated varieties can be considered heirlooms. Unfortunately, the definition of "heirloom" has been somewhat of a moving target recently, but generally it means a variety that is at least 40 to 50 years old, that is no longer available in the commercial seed trade, and that has been preserved and kept true in a particular region. So, for example, if a variety of open-pollinated pepper has been grown in Vermont or Maine for five or six generations and seed has been selected and saved by local growers and gardeners, it would be considered an heirloom variety. Obviously, heirloom varieties have been saved because they have some real virtues. The classic examples are heirloom tomatoes, which often have superior flavor, color, or texture, but lack the holding ability, disease resistance, early maturity, or other characteristics that would make them commercially viable.

Victual Reality

There seem to be broader and more specific meanings of 'hybrid.'

F1 hybrids are the products of selective breeding and produce offspring that are not optimized like they are. It seems that these have been used by farmers since about the 1930s.

More generally, a 'hybrid' is either the result of 'interbreeding between two animals or plants of different taxa' or 'crosses between populations, breeds or cultivars within a single species.' Indeed, it would be hard to carry out agriculture without producing hybrids of this kind.

Given that heirloom vegetables were produced at some point by selective breeding between between populations, breeds or cultivars within a single species, it seems that they are hybrids in the second sense of the term.

a sibilant intake of breath