It's not that you must do side projects or else there is no possible way you are a great software developer. It's that lots of people are just as good as you are and also do side projects. Given the choice, why would you choose someone who is equally skilled but is less interested in the field?

When you are hiring people, you really have insufficient information to know how good they are. You can't see any larger projects they have done or get a feel for how they solve problems that aren't toy scenarios by a whiteboard. Having side projects too look at dramatically reduces the uncertainty in your estimation of their skill level.

So there you go, all else being equal of course you want someone who is 'more into' their field, and all else being equal side projects dramatically improve your confidence in their skill level. So to a competent hiring manager the author of this looks like he has less passion than his competition, and even his skill level is very poorly understood in comparison.