After long discussion we've decided against ruling DC = loss.
Sun Tzu did disconnect, but the opponent team wasn't ahead by at least 60/40 win chance in my book, and it seems others who watched it agree.
We will keep an eye on accounts that DC frequently and if necessary punish them. So: do not hesitate to AR games that result in a DC if you think you deserve the win.
If another admin could weigh in that'd be great.
Just to be clear: we're still deciding for League whether the rule should be DC = loss.