Set by the River question

Hi, I'm listening to a book titled Poker Math by Alex Trott, it states "A player who already has a pair of Aces has a 4.2% chance of hitting another Ace on the flop, turn or river of the hand they are playing. The odds of getting another ace are 22.5 to 1, which indicates a player will get another Ace once for every 23 times they have a pair of Aces. In other words odds of another Ace are 1 in 23. "

I understand the odds and how to get to the odds from the 4.2%. What I don't understand is the calculation to get to the 4.2%

I found this post here (https://forumserver.twoplustwo.com/32/be...) , which states the chance is : C(2,1)*C(48,4)/C(50,5) = 18.4% of hitting a set by the river with a pocket pair.

Perhaps I’m misunderstanding what the book is stating? Perhaps it is calculating for a “specific” pocket pair? Any ideas?