- What you're talking about is working as a freelancer (Freiberufler), which is a subset of a sole proprietor (Einzelunternehmen); these are different business types than a GmbH, and the article is about setting up a GmbH.
- Only setting up a GmbH if you want to "invest money" is overly simplistic. People might choose a GmbH due to limited liability, a more solid shareholder structure, etc. Slightly more generalized, you do have a point that of course a sole proprietorship might be a much better and simpler choice for one-person "companies" who mainly offer software freelancing services.
- Not setting up a separate bank account for your business, GmbH or otherwise, is strongly advised against. Not only by literally 100% of tax advisors, but also by the tax office themselves.