Founder here. Yeah, sorry about that. We totally intend to work with people without visas (it's actually where the idea started). We're just doing a phased launch. Talking to lots of people on the phone is hard to scale. We'll be removing the restriction as soon as we can.
EDIT
People without visas are free to register now. They just can't book phone calls. We'll let everyone who registers know when they can book a call.
Uhm, something weird happened. I tried booking without a visa to see the error, but it let my booking through (I then cancelled my reservation).
Thank you for pointing out the most important answer.
And I assume no remote work allowed as most YC companies stance on distributed teams is hilariously archaic.