Epigenetics make a lot of sense to me, actually. If a plant is subject to cold temperatures while small, it might accumulate coumpounds or grow in a way that makes it less fragile to cold damage. It's likely a trade-off, the plant will be smaller, stunted growth and all, but would be able to resist cold damage better. While the same clone, in a heated greenhouse, would dedicate all resources to growth, yield. I mean, it's pretty well-known that taking greenhouse plants and bringing them outside without acclimatization will be very stressful to them. Plants grow adapted to certain humidity levels, temperature levels, UV levels, etc., and if grown in a stable environment and just instantly switched to something else, shock and death can occur. I've seen this a lot. And there's a lot of pages on "hardening off" tomatoes.
I'm also convinced genetics play a large role as well, though. Genetics is like owning a winter coat or not, and hardening is getting used to lowering temperatures and thus knowing to put it on before going out.
Perhaps tomatoes could be bred to be frost tolerant without needing to be exposed to cold, but the mechanism for that would probably be an allele that just tells the plant it's cold even when it's not really cold, triggering that cold-response with little to no stimulation, so that if cold does happen, it's ready, even if it had little reason to believe it was coming (figuratively, of course plants don't "think").
My grandfather and I have sown tomatoes in fairly cold conditions (~6C), and other than slowed/delayed growth, they did fine. Frost is another thing, though. I had 1 plant that seemingly survived the first fall frost, a few years back, and it was a hybrid, so the reason it survived while the others of the same variety didn't is possibly non-genetic (though assuming that it wasn't bred from double haploids, that variety could also have had some genetic variation on untested traits such as frost resistance).