How is this possible?

Never underestimate Effie Emblem, I guess? He might've been overloaded with their special triggers. It's typically how I've dealt with advantaged gem dudes. Or he might've bailed midway? I can't remember if that still counts as a sweep or not.

It's common practice to surrender when any one of your units die.

Could've just been a meme surrender. I've done that against silly teams before, and gotten wins with healer emblem bc of it. Also, could've been that he lost a unit in the previous match but kept going out of competitiveness/spite (I've done this before as well :p) or lost a unit & won on the same enemy phase.

Also, heyyyy ZekeSinner :)

The hero u see on the defensive win usually have nothing to do with the one you fight, it is just the person main hero.
He might have lost doing a daily AA run.

As others have said, one of your Effies probably killed someone else on their team.

As an aside: when I screw up and lose a unit, I often times finish the map, then start a new battle against a beginner tier team and give them a free pity win. I have like 230 Dueling crests banked right now even after blowing like 15 this week, so I'd rather pop another crest to see some newcomer awarded the points than the constant waive of Cavalry cheesers in tiers 19 and 20.

He screwed his deathless, finished the fight, surrendered next fight to a beginner team.
Usual salt I do as well :)