What about this list of occasions in scripture that mention people seeing God:
- Abraham sees God when God Himself comes to visit Him, along with two angels (Genesis 18)
- Jacob fights a mysterious man and he says after "I have seen God face to face and my life was spared" (Genesis 32)
- Moses sees God's form and glory but not His face— also God tells Moses that no human can see His face and live, yet Jacob says he did so and lived. (Exodus 33-34)
- Job states about God "My ears had heard about You but now my eyes have seen You" and he regrets what he did/said (Job 42)
Isaiah states the follow "woe is upon me for I am undone... for I have seen the King Hashem of Hosts" then an angel proceeds to comfort him by cleansing his sin so he wouldn't fear (Isaiah 6) - Ezekiel maybe sees God, this one I'm not sure. He does state it is the likeness of the glory of God (Ezekiel) so I suppose there is form to it.
- Daniel sees the Ancient of Days, God, with a head of hair as white as wool sitting on a throne (Daniel 7)
These are the only times I can think of that are even remotely related to someone seeing God or claiming so, which took place before John's time, and in the scriptures he likely would have read, so he would know about them.
So what did John mean?
Also, what is God's face? Why is it so important?
If God indeed has a face, is He like a Human?
Side note: I have a theory, just an interesting thought. Maybe it is possible that what was seen as God at the time is not what He was at the time but who He would always be, a human like spirit after He is incarnated in Jesus. Which maybe explains why His name can be translated as "I will be what I will be". So maybe the man on the throne, is a timeless vision of God with a human like body?