And then, straight after, a bunch of new creatures with insane powers were introduced. Are they asspulls? Is bringing new elements into the story inherently an asspull? Is it foreshadowing all that's required to stop something being an asspull, and would Alluka suddenly become justified if it turns out her introduction was just foreshadowing for Ai? How about if, rather than needing Alluka to conveniently heal Gon, Gon was conveniently sick to bring Alluka into the story?
The term is just useless as a form of literary criticism. It's often a symptom of shitty writing (running out of ideas), but if something new is written into the story and used to great effect, it's not a bad thing. A lack of foreshadowing is a minor flaw in the grand scheme of things, there are bigger writing crimes than coming up with an idea a little late. The rose bomb is a good example. It had some foreshadowing but nothing concrete enough that we can be sure it was planned all along, but it was used perfectly and justifies it's existence through that alone.