Well, the chapter implies that he tried to break out with his comparison of the materials of the chair and the Muken. Kubo also established that Aizen could still use his reiatsu because the seals only limited the area he can reach. And he also altered Yhwach's perception of time.
It is not farfetched to think that under such "handicap" he became even more adept with his reishi-kido techniques.
Plus, people forget that Kyoka Suigetsu fused with him – I am trying to say that he is the hilt, and Nimaya also made it clear, "The Blade Is Me"; your soul is your zanpakuto, and shinigami just need to meditate to get more powerful.
That said, I am looking forward to that fight, but I am afraid that Kubo decides that Yhwach will absorb Aizen, or something similar. He is getting absurdly powerful... but I guess that goes both ways.