Could be their crew died and you just missed the message.

ETA: I just noticed you destroyed their O2. That makes it even more likely their crew was dead. Also, you gained scrap from the battle. No one starts with 42 scrap.

^ This. It's very likely the enemy crew died.

^this +1.

You should have received a message when their crew died, letting you know that's what happened. I suspect you have keyboard shortcuts for messages/events activated, and one of those shortcut keys is identical to one of the keys you use during combat. In other words you probably did receive such a message, but did something (e.g. select weapon, fire weapon, select crew member, etc.) immediately afterwards.

It's easy to do in the thick of combat, and it's also easy for this to happen JUST when combat has finished, so that you don't even notice the victory message. A couple of times I accidentally agreed to a surrender in this way (which come to think of it is another possibility in your case here). So now I specifically set events to be controlled by the mouse only, which solves that problem...

There's also an option you can set for this -- if you set the event dialogue to have a brief delay, then you can't accidentally bypass it when you're trying to aim a weapon.