In devlogs the guy stated that he reversed the workflow on purpouse - he wanted to get the final dungeon ready beforehand, so that he won't get the situation of "unfinished final area" because of the lack of time, leaving one of the side dungeons for last.
IF I recall, the first one was 8+ years of "work", but really, the dev worked on it when he had the time/will, doing nothing for some time if he didn't, so it's not really 8x times faster or something. But yeah, he has now a team of people - he, the guy translated as Mii-kun by the google translate that is his engineer and just codes shit, and another girl that is also doing some art and content of the game. I think also music, but maybe they outsource it from somewhere, I'm not sure.