Seeing as you only see ranked games of a person (the majority of the time) and you also do not see if they have been working on another account.... also you do not see anything of the sort of what their main role is or anything that happened in that game.....
There is many factors in which could mean that it is actually the same person. There is no guaranteed one way or another. It could just be that your team is worse and the teams they were facing were better.
There is no certain way to tell one way or the other and seeing as if they get 13 kills and 0 deaths.... of course they will have better farm. That is just an obvious.
Not saying it wasnt actually someone else on that account, im just saying that with the information you provided, and when it was only a single game... there is too many factors that are not hammered out and therefore can not be used to punish the account. That simple. Report them if you wish, that could set them with a mark against their account in multiple games (only 1 is useful for any given game, more than 1 means nothing.... but still can make sure there is at least one by adding in your report with a little description of what is going on).
Also keep in mind that you will NOT always get a notice when someone is punished.