I don’t much mind whether the author if 1-20 claims to be an eye-witness, just that he doesn’t claim to be the beloved disciple.

However, John 1:9 says he was the true light which enlightens every man. “He was in the world, and though the world was made through him, the world did not recognize him.” So dwelling amongst us means dwelling amongst everyone (the world).

John 1:16 – “from his fullness we have all received grace … “. It’s not just eye-witnesses who have received grace; and those who have received grace could be said to have seen his glory.