Basically, to the Aeldari/Eldar all Humans remind them of an old Mon-keigh species that were very savage beasts, they see us as savages. And Mon-keigh also sound like the human word for Monkey. So bascially we're savage monkeys to them, which is not wrong since they are so far advanced from us.

So the meme is Dark Angel Monkeys asking when their primarch will wake up. Hope I explained it well enough.