I've had this before as well, in arena and in Iron twins.

for example in the past in Iron twins, his last hit killed all my team, but as he had brimstone on him it killed him as well and it came up as a victory.

Therefore I think that the victory is down to you achieving the task of killing them, it doesn't matter if your team dies as well, the fact that they are all dead means it's a victory.

That doesn't fit with krama's observation that he had a loss with a double-K.O. before. So everyone dead is not allwyas a victory.

I can't be sure unless someone hands me the code, but it seems to me like the win is basically based on which team dies first, and in case of a mutual team wipe, the last man standing is decided by quirks of how exactly the event sequence is programmed. 

In this particular example, I would imagine that when Leorius does his thing and wipes the team, the fact that Gaius is not last in the team placement order might make the difference. Based on my experience with certain passives (e.g. Seeker or Vogoth), it seems like AoE attacks do damage in a sequence based on the team positions. What we see as one big attack may be 4 separate attacks, dealt to Ukko, Gaius, Kimi and Kymar in a sequence. However, since Gaius dies before Kimi and Kymar does, and his death triggers bombs to explode, that might "interrupt" Leo's attack and actually kill their entire team first, awarding the win to Gaius team where Kimi and Kymar are technically still alive for a brief few microseconds before they die to the rest of Leo's attack sequence. It might not even matter if Gaius is last, as long as his death is not registered before his bombs do their damage.

I could be wrong of course, but it seems consistent with what I've seen. There are many ways that it could've been implemented though, so like I said, I can't say for sure unless I can see the specifics myself.