Well, in my view, I think that the DRAE definition that could apply here is simply this one,
11. conj. Precede a oraciones no enlazadas con otras. ¡Que sea yo tan desdichado! Que vengas pronto. Que me place.
http://dle.rae.es/?id=UkbUarn
Que te lo digo yo. - I am telling you that.
It could also have a bit of the meaning under 6. (denota cierto valor adversativo: Suya es la culpa, que no mía.) as noted by Tomisimo and Angélica ("but" we are more)
In my view, it could be something implied like,
[But realize] That we are more.
I hope I have not messed up your understanding with my comments!
Merry Christmas!