Dear Rabbi Kornfeld,
In a usual instance of Birchas Hash-m, we might have thought that the eidim have to again tear their clothes when they say eidus on the specifics of what the alleged mevorech said. However, why would the shlichim to Chizkiyahu have had to repeat what they heard, beyond simply saying that it happened, which would not have occasioned kriya?
Gershon
Tosfos and the Tosfos ha'Rosh asks your question in Sanhedrin (60a DH Hamelech) and leaves it unanswered. Apparently, there was some need for them to say exactly what they heard.
Alternatively, they did not repeat what they heard, but just described it, and nevertheless there is an obligation to tear Keri'ah even for hearing of such a thing, as some Rishonim write. (See also Perush Kadmon here, Nimukei Yosef, Maharik Shoresh #100)
Yosef Ben-Arza