User blog comment:Xilinoc/Ch. 683 - The Dark Side of Two World Ends/@comment-493809-20160721174524

Yhwach can see every single future. Which means, if there was even a single future in which he saw Aizen's zanpakuto, he will become aware of it - and fulfill the conditions for Aizen to hit him with illusions.

Now, Aizen's zanpakuto is supposed to be gone (I think?), but the big part of Deicide was that he and Ichigo were fusing with their powers. Maybe now seeing Aizen is enough for the illusions to happen, and the sword doesn't matter in the slightest.