Maybe I'm just old and cranky and this is a generational thing, but when did "side project" become mixed up with "side hustle"? I think the dead horse has been beaten already in this thread, with what the author "should have done" in terms of market research, A/B testing, MVP, etc. No need for me to pile on further there. But what strikes me is that the author really seems to draw no distinction between personal side…
That sounds like a work focused side project to me. Things I've made that are personal side projects - iot blind controls (because I wanted it), magic mirror (because I wamted it), triathlon result parser (because me and my so wanted to settle some race stats debates), groupable containerised desktop apps (because I wanted it) etc.
None of my personal side projects set out to develop new skills, they set out to make me or a loved one happy. Often they do develop new skills. My partner is currently stripping and repainting an old bike - shes learning loads, but thats not the intent. The intent is to make it match her other gear because that makes her happy.