It's at least sometimes polyembryonic (I have a few sprouts coming from one seed right now). The seeds are not very plump (relatively good sign as to whether a Citrus is highly polyembryonic). I am not aware of any seedling variation however, but then again planting Citrus from seed seems to be a very rare occurrence here so maybe hardly anyone has grown from seed.
I have a neighbour with a big tree of Lemonades, and I will ask to buy some crop and then sow the seeds, and can give a better idea of how polyembryonic it is from the seedlings.
As far as I'm aware, we don't know with certainty the other parent of the Lemonade. It could be a clementine or one of the other monoembryonic "mandarins" for example (hence higher chance for the hybrid to have formed in the first place).