He mentioned several times that asking for reviews wasn't common (in his opinion) at the time, and so there's little else that could reasonably explain the jump in review count.
I agree that asking for reviews was a lot less common at the time (2012), but he actually says "asking for reviews ... wasn't something that came to the front of mind ... it felt dirty at the time". To me that reads like he was aware it was something apps were doing but he wasn't comfortable with it. So when the CEO of the company asked him "why aren't we higher rated in the app store?" he probably wished the answer was "they're cheating" instead of "because I feel it's unethical to ask for reviews".
Engineer pushback on this was MASSIVE in 2010-2012. There was still a Wild West ethos, so it cost a lot of goodwill to ship the feature. We did it anyways - it was the obvious right thing for the business. Getting there was one of the most heated feature debates I've encountered.