The question stem is a bit tricky here, since it does ask about sufficient justification. However, strengthen with sufficient questions usually ask something along the lines of "the conclusion can be properly drawn/concluded/justified if which of the following is assumed"
However, here we are also asked "The reasoning is flawed because..." This, as you noted, indicates an error in reasoning question. As we have here, we will occasionally get more info in the stem about the flaw. In this case, we know that the flaw is that the scholar mistakenly assumes something without having justified it, and we are asked to figure out what that mistake was.
Oddly enough, I would consider this to be both an error in reasoning and a strengthen with sufficient. Since we are being asked to find what the author missed in their argument, we are doing exactly what we would be doing in a strengthen with sufficient question. We are also finding the flaw in the argument. Regardless of which "category" we choose to put this in, we are being asked to find the flaw (which we do in both a strengthen with sufficient and a flaw question), and then find an answer choice that will fill that flaw (like a strengthen with sufficient). I would not worry too much about how to categorize this question, as we know from the stem that we need to find the flaw that the author mistakenly assumed.
