Yes, they would just need a visitor visa (B1/B2/VWP-ESTA). I think YC is conducting interviews in India mainly for convenience. Why force a 100 potential founders to fly to the U.S. to attend an interview -- which would probably cost for flight+hotel around $1500 per person, so that's $150,000 wasted. Not to mention besides the money, there's time wasted as well -- the time flying in airplane (34 hours -- 17 hours each way). It's simply economically, from both a monetary and time perspective, more efficient to avoid unnecessary travel for an interview.

Moreover, there's also the fact that it's practically impossible to move to the U.S. as a founder of a company (there's no visa for founders). People from a select few countries (that the US has trade treaties with) and who have $100k in savings just lying around might be able to self-sponsor a E-2, but I'm most YC-funded founders probably don't qualify for this either. Especially not founders from India, since India is not a E-2 visa treaty country.

Typically you have ~2 weeks between being notified and interview. Have personally witnessed a lot of teams where one founder couldn't make the interview in the time frame. For the actual YC program both the invitation letter is much stronger, you probably have an established US entity and a strong reason to be in the US. Less likely you get denied a Visa.