I think the sentence is meant to be read: "As many Jews as [the number of] Signal readers" i.e. equating the size of the groups, not saying that they were the same group.
I understand that. It's probably also the reason the date (late autumn 1943) was picked. But in the haste of claiming guilt-by-association and showing a razor-sharp anti-fascist mind to the reader, the author wrote a meaningless sentence, and then fucked it up. That's bad writing.