The point here is one of terminology. The writer is saying that early Christian writers use various Greek and Latin expressions to refer to baptism, and all these expressions imply immersion. He is arguing that baptism was originally conceived as immersion, and that sprinkling came later. "Baptizo" ("I baptize" in both Latin and New Testament Greek) also means to plunge or immerse, as the writer has already said. So "exclusive of" does not mean that all the expressions they use mean plunge or immerse except "baptizo", which means something else. Nor does it mean (though this is not clear from this context) that early Christian writers do not use the term "baptizo", because they do (the writer has already quoted examples). So "exclusive of" means "apart from", in the sense of "leaving aside": if we consider the expressions used, without including "baptizo" in our considerations, we find that they all mean the same.

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Note added at 39 mins (2011-05-25 15:50:22 GMT)
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On consideration, a better way to express the sense of this in English would be "quite apart from" or perhaps "leaving aside".