Perhaps he heard about it when he was a lost boy. Felix called Rumple the “Dark One”. Also, it’s evident Rumple spent time in Neverland as the “Dark One”, so he was already known to PP and the lost boys. Bae was in Neverland for a couple hundred years, so he would know that Rumple was there and he hid so Rumple couldn’t find him, but he heard everything they knew about Rumple, including the castle.
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Don't grow up. It's a trap.