[Svartalfiar]

Doesn't matter what you think.

Not really, but it's a matter of practicality. Widespread across cities, not to mention states, How would one grapevine be the only one that survived? The math simple doesn't work there, unless all the grapes at that time had matching DNA already.

[SunkenCiv]

Speaking of a matter of practicality, it's in the nature of grape propagation that the best vine winds up cloned thousands of times. The alternative is, the DNA matches are just a huge coincidence, or a conpiracy.