I didn't read the article but I have read various of his books. As I did skim the article I know the argument much longer more complicated and far more substantiated that it (perhaps) was in the article. He does use several outside, Roman sources that mention things such as " a man that caused trouble amongst the jews" or "the man they follow which they call Cristus". Stuff like that.
As for only using the bible. That is not a problem from his view point. He does not see the bible as one book or the Inspired scriptures of God. He sees the New testament scriptures as mostly, apart from the synoptic Gospels, as independent accounts of the man, Jesus of Nazareth. To him all, accounts of Jesus are historical documents and he studies them as such. And through cross referencing many, many accounts of Jesus he comes to the lowest common denominator in the all hoo-haa: A preacher/prophet whom denounces the rigidity of his religious leaders and preaches a coming Judgement. He is an apocolyptic Prophet. And since this was one of the major flavors of Judaesim of the time, as Erhman argues, its not at all far fetched to believe in the this man Jehoshua.