It seems to me that the most likely explanation - since there's no notable difference between their campaigns on the surface - is that Elfreth's simply lying, and if she's elected she'll reveal herself as another Sinema or Manchin - another supposed Democrat who will blatantly work for the benefit of the Republicans. And that's exactly why AIPAC (and a number of other groups and individuals who customarily support Republicans) are supporting her - to them, it's not seemingly arbitrarily one Democrat over another more or less identical Democrat, but a fake Democrat over a real one.