This claim isn't necessarily true. The article mentions that in 1.3 billion years the earth will be uninhabitable for most organisms and that in 4.5 billion years the sun will engulf the earth. That means that we could have up to 4.5 billion years because we don't know if in 1.3 billion years humans won't be able to live on the planet. Also, there's no telling what can happen between then and now, and there's a possibility that humans can become extinct or that the planet will be uninhabitable sooner.