This most be a bug (Yay for more bugs) or some kind of best defense EVER!!!
I think the casual PVP bug is still in effect. Any time your defense team loses in casual PVP, you get rewarded for a defense win in regular PVP. If this player put up a 4x Firebomb team and then only played casual PVP, I could imagine them getting farmed enough to have that kind of stat-line.
Some of us have been complaining about this for months (maybe over a year now?), but it has never been fixed. It just makes no sense that anything that happens in casual should impact ranked stats or rewards.
Edit: here is a very similar discussion from 6 months ago. If you read down a bit, you’ll see that the topic was old even then…