Really? Huh... Because other books, seemingly reputable, have republished the old fallacy of 2x 20mm MG151/20s in the cowling for ages.
No, the mistake they made was that the 109K-4 had 2x MG 151/15 in the cowling, but its repetition owed to their not doing real research with primary sources. What's his name, William Green(?), started that myth and it appeared in all sorts of craptastic airplane books for decades. IIRC, even Green realized his mistake relatively quickly, but too late to contain the damage.
This is a book that uses primary sources, e.g. RLM records, number blocks, unit records, and photographic evidence, to support its claims. They
never cite other books about the 109.