Unless by "the Father" he meant IAO (the Monad), not Yaldabaoth (the demiurge).

It's essentially the same thing as claiming "All is Brahman, Brahman is All." In other words, "I (and you, for that matter, if only you realize it) am one with Brahman, an emanation of Brahman, a mask that Brahman wears."

If you're still not convinced, I can go into the Kabbalistic associations between Kether and Tiphareth. Or even the Yechidah and the Ruach, or the Shekinah and the Yechidah for that matter. In the light of those concepts, it's not outlandish that a Hebrew mystic would claim to be "One with God."

But "God" in this case refers to the unknowable ("ineffable") infinite unity (monad) beyond the abyss of ignorance (Daath, knowledge or Gnosis, beneath which everything takes place, and through which none can see but a reflection of what's below; i.e. a projection of one's own subconscious, thus man makes "god" in man's image, and is deceived by it. That is the demiurge which masks the imperceptible monad above, knowable only by aspiring to the pre-eternal unity within, which is the source and substance of everything that lives (unknowable to the Guph and the Nephesh; knowable only to the Ruach which itself is inscrutable to the Nephesh and Guph, and must first be sought from within before even attempting to peer into/beyond the abyss without going insane)).