It's definitely not private information. It might not be information that everyone needs to know, but I think that anyone who would be impacted by your reduced hours has a need to know about it.
It might have been better if your boss had just told everyone the hours you'd be working and let them figure out it was fewer than before. On the other hand, stating it the way he did made it clear that it was not a decision you made based on personal reasons, nor was it disciplinary in any way. It allowed him to say that it was because of the department's needs, and to thank you for it.
The biggest mistake he could have made is telling people who wouldn't have any particular need to know, but telling your whole department doesn't necessarily mean he was telling people who wouldn't need to know what your hours are. It depends upon the size of your department.
But regardless, I wouldn't worry about it. It feels weird to have it announced, but that's the most efficient way of letting people know about changes in other's jobs.