I "retired" about a year ago after almost 25 years in the industry. My story since then is a lot like that of the author of the article. I've written more code this year for love than I bet I did in 10 years for pay. It's been exactly the retirement I wanted: one of the most productive times of my life.
That's if you're like me, and you find this question really annoying to explain, and it gets asked constantly.
"Self-employed" sets off a lot of bullshit detectors, because...well, lots of bullshitters use that one.
Or maybe just keep saying programmer, if that's what you're doing. No need to explain in detail how the compensation works (or doesn't exist).
"I retired from corporate, doing my own thing."?