Instead of an excess of planning it sounds more like the author approached his startup without any real plan whatsoever.

After attributing failure to spending three months on mockups and market research before doing any actual development, the author concludes with:

After the past experiences I don’t plan anything. I don’t even do market research... We just jump in and start developing, after doing some sorta brainstorming(what features we should add, etc). We ask a couple of target users what they think and decide whether we’re gonna add that feature or not.

No, you ARE still making plans, they just happen to be ones that are better and more appropriate.