The once-autonomous individuals who now stood before Tang Yin as slaves had been stripped of their capacity for independent decisions. They were bound to obey his every command without hesitation.
While their individual thoughts and emotions still existed, the Heaven Binding Seal etched within their souls left them unable to mount any form of rebellion or decline Tang Yin's orders.
Xu Yun's prior ability to exhibit childlike behaviour and harbour suspicion toward Tang Yin while under the influence of the Heaven Binding Seal in the Thousand Blossom Pavilion was due to Tang Yin's decision not to fully activate the Heaven Binding Seal at that juncture.
Tang Yin hadn't immediately enacted the full influence of the Heaven Binding Seal upon her, as he required her to lead her family to Broken Tooth Mountain. As such, while Xu Yun was indeed subjected to the seal's effects, her free will hadn't been immediately revoked.