As a writer, I don't think it's that hard to accidently pull general ideas and similar phrases from other sources without attribution, particularly ideas that are fairly commonly held. In some cases it's very hard not to restate a factual point in a markedly different manner than the last five sources you have reviewed.
However, verbatim, direct quotations are another matter unless he planned on referenceing the source and it just got lost in the editing and rewriting process -- which I suppose could happen without too much trouble in a massive project. Still, not very good for his reputation as a historian and writer.
Charon