This article is a little bit one sided. So what if most side projects never succeed? You can always learn a lot by attempting to do a large scale project yourself. Most real-life projects are large scale legacy projects that you are simply thrown into. Most likely you don't even care about them nor you can make any sort of decisions.

By doing side projects you are being creative. You learn new stuff. You actually care about your project. You also have to do everything yourself, including planning. Where else would you get such experience?

And the net result is that you learn new stuff in your side projects, something your employer gets for free. Your passion will pay off, eventually. It is nearsighted to assume that just because you "waste" time on side projects you are hurting you employer.

You hurt them more by simply not caring enough about the technology nor the project.