Yandex might be the most popular search engine in Russia, but also they're by far the most popular logistics company for private people here. They do deliveries (food, groceries, other goods), taxis, moving vans and so on - all strongly Yandex branded. If you walk around the streets of Russia's big cities, their brand is absolutely omnipresent (and also in some former Soviet countries, like Armenia).
Eating into their business would require much more than source code, but of course an analysis of the code could lead to finding more security issues.
> potentially a boon for open-source development?
That'd be an absolute copyright/licensing nightmare, just because the code was stolen and published doesn't mean it is now "open-source".