What's not true is that this is the only screen that matters. It's true that our world is being filled with more and more screens. What's not true is that all these new screens are phones.
Along these lines, what’s always been the most important thing to consider is the context of different screens and building the right experience for that specific interaction.
It just so happens a lot of people right now are trying solve for experiences that arguably should only exist on mobile. At least for an MVP product. Instagram unveiled a web presence much later, but their product is still primarily consumed on the phone.