There had to be a massive change in development for the game beyond switching from a dlc to full game for this timeline to make sense.

Either they built a lot around a feature that ended up not working or being scrapped, or the game as a whole went through some type of mid development crisis that made them need to reboot/scrap a lot of work.

God of war ragnarok is gonna end up taking less time than silksong to release.

And this is why kickstarter can never be trusted, especially the stretch goals. Yes hollow knight is great and silksong seems to be more than originally promised, but the point stands. In december 2014 fans backed the game and hit the stretch goal for a second character. 8 years later, 7 years after the original planned release date and 5 years after the actual release date of the game itself, there is still no second character/now a game with that character. Imagine how long they wait would have been if they hit that third character.