Any ace, regardless of the second card, is 52% or better to win against a single random hand. These are just numbers to help get your head around starting-hand requirements in heads-up versus a full ring. If you pushed all-in every time you had an ace heads-up you would not win 52% of the time or more.
The reason is simple: you don't get a call every time you push. You are almost guaranteed to get a call when the person has a hand that seriously dominates your own, and a fold when they have junk. The numbers in this article are just one way to help make you comfortable playing at the aggression level needed to dominate heads-up poker.
The better a player you are, the more aggressive you can be without being reckless. The more aggressive you can be as a heads-up player, the more often you'll find yourself winning the match.