The issue with encyclopedias (or encyclopediae? I think it has some kind of weird plural? w/e) is that they're written by people who don't necessarily have a detailed expertise in a single aircraft line; as a result they're usually correct in a general sense but they get details wrong a lot.
That book is no exception, as it was conveniently available in PDF format with a quick Google search, it became clear that it was susceptible to several myths and legends concerning the 109, particularly the later models.
Curiously the neither non-jettisonable drop tank nor the MK 108 pack is mentioned in Prien and Rodeike's book on the F G and K series, which dedicates about as much paper to the G-10 as that book does the entirety of the 109 series, nor any other book I own on the series, nor any internet source, even Wikipedia, leading me to the position; I'll believe it when I see it.
Perhaps he misinterpreted Ruestsatz 4, two MK108's mounted in underwing gondolas a la Ruestatz 6 (the one we have in game), however this was never used operationally. Not really sure where he got the external tank from, I've never heard of that.