A Zanpakuto's Shikai could be released directly without speaking its release command if one's power had reached the level of Bankai. The reason Soul Reapers still called it out in battle was probably, first, a habit formed over time—after all, their main enemies were Hollows with little intelligence—and second, perhaps to bolster their own confidence. Words were also a kind of power.
For Aizen, performing Kyoka Suigetsu's release ritual in battle was an incredibly simple matter. The reason he had not used it was not because he could not, but because he did not want to. Aizen was an exceedingly proud and confident man. One only had to look at how he had not directly killed the Visoreds, Kisuke Urahara, and the others who had fallen under Kyoka Suigetsu's influence back then.
Of course, that was not what allowed Shen Fei to disregard Aizen's Kyoka Suigetsu. What truly let him ignore it was Enrakyoten. He could now use Kyoka Suigetsu as well. Even Tokinada could rely on Enrakyoten to dispel Kyoka Suigetsu's effects, much less him.
Fighting Aizen was not merely a contest of strength; it was also a contest of wits.
"What is going on? Could they have betrayed His Majesty?"
The Quincies who arrived afterward were shocked to find their companions fighting one another. While it was not as though no Quincy resented Yhwach, almost none dared betray him—at least, not while Yhwach was alive.
Yhwach was cruel to his enemies, and equally cruel to his own people. Once someone failed, they no longer had any value to him. Several capable Quincies had not died at enemy hands, but at his.
To put it bluntly, Yhwach himself was responsible for most of his defeat. With one Auswählen, he threw away every advantage held by the Sternritter, causing the