If you were able to "no longer be a night owl" after changing diet, through exercise or by other changes in your behavior, you were in fact never an actual night owl but simply someone with bad sleeping habits. You read the same thing whenever there's a discussion on clinical depression. People push their anecdotes about mindset changes but fail to acknowledge they were never clinically depressed in the first place.
I have a very hard time believing that this stuff has to do with anything other than having the discipline to have a proper sleep schedule. It's possible to adjust your sleep schedule. People do it all the time when they change timezones. I didn't read the article, I'll go read it and update if I change my mind..
For me, it was about establishing new habits, setting up an environment conducive to sleeping by midnight, and having some external pressures like a work schedule, etc. I've heard this validated by external testimony as well.
Can't say there are no biological factors at play that impact individual inclinations, but it's a bit hard to believe that we're hard-wired for one over the other.