He meant that if your goal is to make money with a business, then you should start with businessy stuff like figuring out if there's a market for what you want to build.

You're free to start a business by building the product of your dreams, but no matter how much you personally like it, there's no guarantee people will pay for it. In other words, if you want money, but start with building a product, you're likely to have a bad time.

But if you just want to build an awesome product, and don't care about whether you make any money with it, then there's no problem.