You can see in the screenshot of it being the number one restaurant that it only has 83 reviews (likely the originals from friends that likely got him into the top 10,000), yet it's still #1. I'm speculating, but TripAdvisor likely ranks not just by reviews, but probably page views and other interactions that one could consider to be viewers having a positive judgment of the restaurant. In fact, you can see from the other screenshot, the page has the same 83 reviews when it was ranked #156, so it got to #1 without receiving any new reviews.