I would say they would start with a website, for a few reasons:

- You need to build a web platform for mobile apps to communicate with, you might as well build a frontend for it too.

- Responsive frameworks and technology is very advanced, you could go with a mobile-friendly approach and cover many devices & browsers from day 0.

- Easy to validate, gather feedback and tweak.

- Once they have traction and a large user base, start targeting specific mobile platforms and build apps for them. You take the guesswork out of defining what/if yo build a mobile app.