You can report him to Nordeus with the screenshots you took.
You're right when you say that tank it's not cheat, but when I asked Nordeus about tanking, their answer was:
"- Yes, you can put your team to loose and tank, but you can't deliberately put your team to win or lose against specific opponents, because this way you are manipulating the league classification, and that it's not allowed since it's not "fair play".
So, go ahead...report them to Nordeus.