It's like you don't remember E3 flash sales, holiday sales + every so often got big discount. The game was on top during these months due to that. No need to be an insider, just observe Amazon and NPD threads.
If you just wanna point me as a downer, then do so. Fact is the game sold really well. Fact also is that legs are also due to heavy discounts and bundling.
At the E3 sale it was discounted by $20, I wouldn't call that a "big" discount as it's just a regular discount that every game tends to get after being out for a few months. http://www.playstationlifestyle.net/2017/06/13/playstation-store-sales-north-america-week-june-13-2017/#/slide/1
It wasn't until a few months ago that it got a "heavy discount": http://metro.co.uk/2017/11/23/playstation-store-black-friday-sale-has-horizon-zero-dawn-gt-sport-and-more-at-50-off-7104201/
But this isn't unusual, if you can say one game is selling because of discounts then we may as well say all games are selling because of discounts...Seen as all games get discounted at some stage.
Also the bundle argument is hardly worth discussing, it's not like you only get 1 bundle to choose from.