You were correctly denied access since you were departing on a flight that is not operated by a OW Carrier - what you arrived on is irrelevant. Lounge access based on status only applies where flight is both marketed by and operated by a OneWorld Carrier

If you had been in business class, then you would have been entitled to whatever lounge is provided to those in business class

If you had been in economy class on a flight both marketed by and operated by a OW carrier, then you would have been entitled to the lounge for those with OW Emerald status, but not the AM lounge