Clearly a possibly amount since he did it.

Would it be possible for someone who was not supporting themselves that way? I.e. for someone with a 40 hr / week job?

I don't know about possible, but I'm sure the Michael Phelps regimen is wildly impractical for everyone, including Michael Phelps.

My personal experience is that "outrunning" a poor diet is absolutely possible, and I kind of did it unintentionally. I started biking to work out of necessity. I wasn't planning to lose weight, but it happened. I had no weight loss ambitions or plans, but it was probably a good thing.