AFAIK it won’t destroy all the vitamin c if you only heat it for a short time, but they didn’t do “cooking for a short time” at the time, in part because you need to cook for longer to kill all the germs. Dehydration typically means either oven (often even hotter than boiling) or drying in the sun, and UV rays are generally pretty descructive so I wouldn’t bet on that method either (sailing ships probably tried raisins at some point, though it’s pretty much down to luck whether they figured out that it helps against scurvy (in the hypothetical case that raisins contain sufficient vitamin c)).

There’s quite a few preservation methods that don’t involve heat, though, like submerging in salt, vinegar, oil, sugar and indeed alcohol.