It is widely known that the Qur'an as recorded in writing is not in the order of its revelation. However, a common misunderstanding is that any single numbered verse would have been revealed at the same time as a single unit. That has not always been the case.
Before looking at 33:33 we can examine the famous verse of 5:3 to see an example of this phenomenon.
- You are prohibited carrion, blood, the flesh of swine, and what has been offered to other than Allah, and the animal strangled or beaten to death, and that which dies by falling or is gored to death, and that which is mangled by a beast of prey—barring that which you may purify —and what is sacrificed on stone altars [to idols], and that you should divide by raffling with arrows. All that is transgression. Today the faithless have despaired of your religion. So do not fear them, but fear Me. Today I have perfected your religion for you, and I have completed My blessing upon you, and I have approved Islam as your religion. But should anyone be compelled by hunger, without inclining to sin, then Allah is indeed all-forgiving, all-merciful.
The first and last parts of this verse, discussing food law, were revealed a year or so earlier to the highlighted portion that was revealed in 10 AH. It is commonly called the Ikmal verse, which, as you can see, is actually a part of the Qur'anic verse 5:3 rather than the entirety of it.
So in a similar manner 33:33 is also made up of parts that were not all revealed together.
- Stay in your houses and do not flaunt your finery like the former [days of pagan] ignorance. Maintain the prayer and pay the zakat, and obey Allah and His Apostle. Indeed Allah desires to repel all impurity from you, O People of the Household, and purify you with a thorough purification.
The Qur'anic commentaries from all schools of thought in Islam demonstrate that the second part, famously called Ayat al-Tathir, was revealed separately and distinctly from the previous portion of 33:33 and also the subsequent verses.
Just like the mention of food law in 5:3 shows the continuity of the food law theme before and after the completion of religion theme, so also here the use of the all-feminine gender in the grammar of the part of 33:33 before the Ayat al-Tathir and afterwards in 33:34 onwards shows that those sentences were on the theme addressing the wives of the Prophet (s).
The Ayat al-Tathir part, on the other hand, uses the masculine gender that is grammatically appropriate when addressing a set of people that contains at least one male.
The reference to the Ahl al-Bayt in 33:33, therefore, is not a continuation of the address towards the wives of the Prophet (s), but an independent theme addressing 'Ali, Fatima, Hasan, and Husayn, on them all be peace, as the chosen Family of the Prophet Muhammad (s).