You do not know that oxygen wasnt present in the atmosphere billions of years ago
Oh, but we do know that there wasn't free oxygen in the atmosphere until about 2000 million years ago. There are deposits of uraninite and beds with water-laid pyrite that could not have formed under an oxidizing atmosphere; the banded iron formations in Minnesota (and many other places) formed from dissolved ferrous iron in ancient seas, also an impossibility under free oxygen.
But if exclude oxygen from the atmosphere, you get no ozone, and without ozone you have no ammonia.
Jupiter has ammonia by the cartload, but no oxygen or ozone. Your sources need to check their freshman astronomy.