That’s a bit strange because, well, the vocabulary only makes for one part of a language.
It completely leaves out things like phonetics or most importantly grammar.
If Yiddish isn’t written in Hebrew letters most Germans can read it. And hearing Yiddish (if you still can) is usually closer to modern high German attuned ears than say Suisse German.
What also feels construed is the part about the diminutives: The Yiddish Hershele example is exactly how people form diminutives in south west Germany till this day, exactly the area where Yiddish originated. Yiddish diminutives are such a direct pointer towards their German origin, that most non-southwest Germans that don’t use these diminutives would read the Yiddish ones and think a Swabian/Hessian/Palatine/Badisn wrote them.
The same holds true with regards the „Yiddish Slavic syntax“: Unlike what the author claims multiple negation is still used in south west Germany (where Yiddish comes from) as well as Bavaria.
It’s either intent (bad, but normal as the field of linguistics is often highly politicized - is it a coincidence that Chomsky studied this?) or just lack of looking at the „German component“ because it is less exciting than exploring the „Polish component“ (understandable).