re: Does anyone actually believe this

EDIT: The earliest surviving Greek manuscript that contains the Testimonium is the 11th century Ambrosianus 370 (F 128), preserved in the Biblioteca Ambrosiana in Milan, which includes almost all of the second half of the Antiquities. -- And this is why most people are uncertain of which are interpolations. Early Christians are the only sources for which actually copied what he wrote, and they all had, shall we say, a divine conflict of interest.

James Dunn states that there is "broad consensus" among scholars regarding the nature of an authentic reference to Jesus in the Testimonium and what the passage would look like without the interpolations.

James D. G. "Jimmy" Dunn (born 1939) is a leading British New Testament scholar who was for many years the Lightfoot Professor of Divinity in the Department of Theology at the University of Durham, now Emeritus Lightfoot Professor. He has worked broadly within the Protestant tradition.

Dunn has an MA and BD from the University of Glasgow and a PhD and DD from the University of Cambridge. For 2002, Dunn was the President of the Studiorum Novi Testamenti Societas, the leading international body for New Testament study. Only three other British scholars had been made President in the preceding 25 years. In 2006 he became a Fellow of the British Academy.

In 2005 a festschrift was published dedicated to Dunn, comprising articles by 27 New Testament scholars, examining early Christian communities and their beliefs about the Holy Spirit. (From Above)

Even Christian scholars admit that it was augmented or had some things omitted, brosky.