The secret to designing entire applications in your head is to be intimately familiar with the underlying platform and gotcha's of the technology you're using. And the only way to learn those is to spend a lot of time in hands-on coding and active study. It also implies that you're using the same technology stack over and over and over again instead of pushing yourself into new areas. There's nothing wrong with this; I actually prefer sticking to the same tech stack, so I can focus on the problem itself; but I would note that the kind of 'great developer' in view here is probably fairly one-dimensional with respect to the tools they use.
