This question deserves a better answer than mine, as I am not familiar with Abraham Polak's work and whatever linguistic evidence he may have offered (if any).
However, it is really difficult to imagine how the heavy influence of Middle High German on Yiddish could have been established without massive language contact. In fact, Yiddish was occasionally called טײַטש (taytsh) during the 12th century, i.e., the name used to denote Yiddish, at the time, was a variant of the contemporary endonym for German.
I believe that current sources tend to offer a rather different story:
Before the Revolution of 1917 the Krymchaks always considered themselves true Orthodox Jews, although different from the Ashkenazim, and they were also seen as such by other Jewish communities. Up to the period before World War I, the Sephardim of Turkey served the Krymchaks as a reference group of higher status and provided an authoritative religious tradition. The Krymchak attitude toward the Ashkenazim settling in the Crimea was more ambivalent. In daily life, the Krymchaks sometimes had negative attitudes toward them; however, they admitted that the Ashkenazi Jews were more cultured and educated. In the past many Krymchaks knew Yiddish, and even now one meets some Krymchak elders who understand it or even speak it. In the cities where the Krymchaks lacked communities of their own, they joined communities of the Ashkenazi Jews and attended their synagogues.
The Krymchak language is a language distinct from and unrelated to Yiddish. It was written using the Hebrew alphabet until shortly after Polak's emigration from Ukraine, and apart from the different script it was just a dialect of a Kipchak Turkic language with no Germanic heritage, but with a definite linguistic connection to the Khazar empire.
I have no evidence showing that Polak failed to distinguish Krymchak and Yiddish, both of them used by (geographically) overlapping groups of people in Crimea in certain periods of time. However, if he hypothetically did, he might have arrived at his conclusions without too much additional bias.
I understand Polak to be a historian by profession, rather than a linguist, and I believe he probably lived rather close to Crimea until his age of 13 but not since. A person with deep intellect and amazing analytical capabilities like Polak was might perhaps slip in connecting their childhood knowledge of the 20th century liguistic situation in Crimea with their professional research into the Khazar empire which existed more than a thousand years ago, predating even the presumed independent birth of Yiddish in Middle Europe.