grand arena score or possible cheat

You'll have to see when the history comes out but undermanning teams and/or trying to end with full health and protection give more banners. Not totally crazy to think they made it up that way. 

If they can clean up with a solo character and that character finishes with fulll health and protection, they'll get 45 points for that.  So, it is "possible" that they got this score.  My opponent dropped two battles and got an 1809.  Each team had one character left and they 45'd each one.  A third battle would probably be 35 points max?

But, like ATM said, check the history to see what they did as it's probably be valid scores.

I'll wait for it to drop on swgohgg then I'll know for sure