In their Q&A thread the other day I believe they said that helium price isn't likely to impact them all that much in the short term. Maybe they'll use hydrogen at some point in the future, but it seems like they don't think it's necessary for their business model to work, given the relatively small size of their airships.

We aren’t running out of helium in the short-medium term but it’s still definitely a limited resource and there is a legitimate concern about losing access to terrestrial helium once we’ve extracted all we can. Unlike fossil fuels, helium isn’t something we can synthesize from elements that are abundant on Earth.

Here's hoping we get that fusion reactor problem solved.