as Salaamu Alaikum wa Ramatullahi wa Baraktuhu
I find that many non-Muslim seem to have the concept that the Qur'an is a set of Commands and each ayyat is to be taken alone. The result is it is nearly impossible for a non-Muslim to underatand that the Qur'an is 114 seperate books, each with a specific message. An ayyat can not be taken alone and understood. To understand one must know the reasons a Surah was given and the goal of the Surah, from there one needs to examine how an individual ayyat relates to the message of the Surah
A tool that helps is to study the available Tafsir
Surah al-Ahzab is very complex and was revealed at a time of great hardships on the Sahabah.
To avoid confusion I am going to Quote from the Tafsir of Maududi
The Marriage of Hadrat Zainab became the subject of many lies against Muhammad(saws) and Islam and even against Allaah(swt) It was a very trying time.
It was a very difficult time. To understand what the Qur'an is telling us we can not simply read ayyat 50 at a minimum we must read ayyat 36-55 that gives us more insight into the events.
For the Tafsir I Quoted
SOURCE
I am not knowledgeable to give any Tafsir, but my personal opinion of this it is not that an exception was made for Muhammad(saws) but a command he was forbidden to divorce his wives as the needed special protection as having once been married to him, they would not be eligible for remarriage. They required more protection than other women, as a result Muhammad(saws) was forbidden to divorce them after the surah limiting wives to 4 was revealed. Notice that after the revelation was given he did not take additional wives. But the wives he had he was forbidden to divorce. Just my opinion Astagfirullah