Such a greenhorn...
He said "I 've been a member for twice as long as you and have a fifth as many posts"
Which is saying that Nicholas has 5 times as many
If he had said "and YOU have a fifth as many posts" then you would have been right to correct him.
But he didn't
So you're not
I just looked at it again and he did say "and you" twice which does make it more confusing.
but i think he just made a little mistake on that.
and you still weren't riight
See my second post which addresses pretty much everything you just said