I have the same omnibus (or at least one that relates the same story) and I remember it the same way. I dont have it in front of me either but I dont think your memory is off.
How true the story is......well you never know but I dont see any reason to doubt it either, really. Even if Jimmy was selling well shortly before Kirby came along, books do occasionally just drop off the charts. Especially back when the market was more flexible and fluid.