Aizen’s illusion on Yhwach worked because Kyoka Suigetsu had already put Yhwach under Complete Hypnosis , so Aizen could make him see and react to a false reality instead of the real battlefield.

How it worked

Aizen’s Zanpakuto doesn’t just create simple illusions; it controls the target’s senses so thoroughly that they can be tricked into seeing, hearing, and judging the situation incorrectly.

Against Yhwach, the key point was that Aizen had previously made contact and established the hypnosis effect, so even later in the fight Yhwach was still vulnerable to Aizen’s deceptive positioning and appearance.

Why Yhwach was affected

Yhwach’s power, The Almighty, is extremely dangerous because it lets him see and alter possible futures, but Aizen’s hypnosis interfered with what Yhwach thought he was seeing in the present.
In other words, Yhwach could read the future, but if the “current” scene was already being falsified by Kyoka Suigetsu, his decisions could still be pushed off course.

Simple example

Think of it like this: Yhwach was looking at the wrong chessboard.
Even if he could predict future moves, Aizen had already made the board itself look different, so Yhwach could be tricked into responding to a fake threat or a fake opening.

Important nuance

Aizen’s illusion did not “beat” Yhwach by itself. The final defeat of Yhwach involved multiple factors, including Uryu’s intervention and the broader endgame around Ichigo.
Aizen’s role was to disrupt Yhwach’s perception long enough to create openings, not to fully overpower The Almighty on his own. TL;DR: Aizen’s Kyoka Suigetsu fooled Yhwach’s senses, making him misread what was happening in the present, which weakened the usefulness of The Almighty and helped set up the final defeat.