I too thought upon reading the translation (thank you so much, Pikari and Celesse!) that here is a way out of the "trouble of two Syaorans, one of them heartless"!
For the timing of the heart-growing is not exactly ties to the breaking of the seal - when the latter happened before the first had, it does not mean necessarily that he cannot grow the heart. However it makes sense that he would have to grow it soon, before the C!Syaoran nature can overwhelm him.
I see a hint of hope, despite the creepily fluffy
ending of the chapter, for Syaoran-turned-C!Syaoran.