Author here. I don't work full time but I'm a master's student. I had a decent enough chunk of free time to dedicate to this without too many serious obligations. It's hard for me to estimate time per day, as there were some week-long periods where I did very minimal study when particularly busy, and other periods where I'd spend 20+ hours a week of solid effort on it. I think achieving this is probably outside the scope of someone who isn't extremely abnormally self-motivated and efficient and who has a family, long working hours, etc. That being said, I think most people can find the time to reach a moderate conversational level in a year without too much trouble, assuming they work efficiently.
It's one thing to imagine yourself doing this given six months of dedicated time (not that they claimed to have had this) and another to get a real sense of how one might fit this into one's life. Anyway, great, detailed article and an impressive accomplishment.