"I am wondering why surplus aces merits a hit instead of a double in the 10v9 situation. It cannot give the dealer a soft hand, and an ace would help you just as much as it helps the dealer. Perhaps in addition with the quantity of 10s, it will create more pushes rather than wins?"
Imagine that you are in a DD game and there are 8 ACES remaining to be played with only ¼ deck remaining.
Of the 13 cards, 8 of them are Aces. What happens if both you and the dealer get an Ace ? You win. You don't push.
More realistically, for the situation of 10 vs. 9, I simply specified that it was a close decision, a fairly low count that means.
The T.C. when adjusted for the extra Aces makes the T.C. positive and an easy double.