All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
Some scholars stated that answering the Salaam in writing is the same as answering it orally, which means that it is obligatory to answer the written Salaam. But it is not obligatory to answer in writing; rather, it is permissible to answer orally.
Ibn Hajar Al-Haytami said, “As for the addressee, he has to answer immediately. If the greeting of Salaam was sent to him by someone else, then he is obliged to answer orally. If the greeting with Salaam was in writing, then he must answer either in writing or orally…”
So it is possible that the Mufti answered the greeting orally.
It should be noted that there is an automatic reply system on our website that answers with the Salaam to the questioner and tells him that his question, number such-and-such, was received.
Allah knows best.