Ace looked at where the fifth curse user was hiding and subtly waved in that direction, ignoring Ava, who was growing impatient with every shot she missed to hit Ace. In her frustration, Ava missed his subtle signals. The reason Ace was so confident that the unknown curse user was Matthews was that a person couldn't enter a curse domain without the domain master's knowledge, even if it's an artificial one.
One can invade and hide in another's domain if their curse abilities allowed it, but their entry into the domain would be revealed to the domain master. This was just how a curse domain functioned.
However, the fifth user was able to enter the artificial domain without Sonia's knowledge meant that the artificial domain disc EAD Matthews gave her was rigged. So, the odds were that the intruder was no other than EAD Matthews, and Ace was willing to bet on these odds.
