I don't know why, only, that this is what he did write. But I don't have any issues with that.
Maybe it wasn't a figure of speech, and he actually meant "we," in the absolute sense.
I'm getting a bit of a flashback here about literal interpretation.

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You've got to work with the way he wrote, whomever the writer was. Are you actually thinking John meant that he was saying that he could be one of the liars who did not actually have fellowship with God?
If you don't take into account writing styles and figures of speech and such, you can end up in the weeds.
1 John 1:3 KJV
That which we have seen and heard declare we unto you, that ye also may have fellowship with us:
and truly our fellowship is with the Father, and with his Son Jesus Christ.
1 John 1:6 KJV
If we say that we have fellowship with him, and walk in darkness,
we lie, and do not the truth:
How do you reconcile these?
Is he assuring us that he truly has fellowship with God, and then turns around to say, but maybe I'm lying?
Many passages are written in such a way that they address the one who professes faith without making a statement of whether that faith is true or not, rather, by showing how we can know the difference.
Later in John's letter he'll write that those who leave us were not of us. They claimed faith, but they left, showing their faith false, as one example. He didn't say, You have many false among you, but he did say, those who leave us were not of us.
Paul wrote a lot like that also, allowing people their profession of faith, but giving the signs of true and false faith. I can show you examples if you like.
Much love!