Good question, Zazaban.
The Imams were like "Guardians" of Islam. Imam Husayn was not a prophet, at least I don't see that quote as saying so. Baha'u'llah has distinguished Imam Husayn from the other Imams, but I don't think that necessarily means He is saying that Imam Husayn was a prophet.
However, Aziz Mboya, author of "A study compilation on the Tablet of the Holy Mariner," states otherwise. Based on a passage by Baha'u'llah in the Iqan
(XXXIX), where Baha'u'llah states "How bitter the humiliations heaped upon Me, in a subsequent age, on the plain of Kárbilá!
[Imam Husayn]", Mboya writes:
"The passage below about the suffering and martyrdom of different prophets when viewed from the "station of pure abstraction and essential unity", indicates Imam Husayn was a prophet (dependent/lesser)—the one who was martyred in Karbila." (http://bahai-library.com/study/mariner/imams.html
But I don't believe this to be the case. As you know, Baha'u'llah has designated Himself the station of "Sender of the Messengers." He has "sent" Imam Husayn by way of Muhammad. Imam Husayn was working under Muhammad as a guardian of Islam, but that does not distinguish him as being a prophet. Being an agent of Muhammad, his station was lower that of the Prophet's. The Manifestation is a mouthpiece of God, and such a passage in the first person really consists of the Words of God; that is, when Baha'u'llah writes "I," the passage isn't referring to the man Baha'u'llah, but to God.
Mboya tries to make the case for the Imams being prophets by using both hadith and the Baha'i Writings. For example, he quotes Imam Ali in his "Sermon of the Twin Gulfs" as saying:
"I am the First and the Last! I am the External and Manifest and I am the Internal and Hidden! I am with the Cycle (kawr) before the Cycle began! I am the Companion of the Dispensation (dawr) before the Dispensation started!"
But again, all prophets, apostles, etc., are all mouthpieces of God; God speaks through them. When Imam Ali says "I am the First and the Last" etc., he is not referring to his own person, he is just working as a channel through which the Words of God are conveyed to us.