The following could be aspects of interpretation:

 

Perhaps the merit of chaining Satan was not in effect by that time; for that incident took place in the second year of Hijrah.

 

Chaining Satan does not necessarily mean that they have no influence at all. According to a considerable interpretation of such chaining, the meaning is only to weaken their strength and limit their abilities so that they become unable to act as they do outside Ramadan.

This thus gives a great opportunity for those who want to make benefit from Ramadan. As for disbelievers and ungodly Muslims, they do not enjoy this merit and are easily influenced by the Satan, even in this weak condition.

Allah Almighty knows best.