A trick to avoid AFK/leaving the game penalty?

I played an ARAM game around 12:20AM, 8/7/2018, and there was an Ekko in my team. He played about 3.5 minutes, and got killed 5 times, and then rage quit. The weird thing is that, he came back around 3 mins later, and watched us losing the game, which ends around 20 seconds after he reconnected.

When he rage quit, I talked to the other teams "our Ekko rage quit, so please end quickly". And one player in the other team said that he just played with the Ekko 10 mins ago and Ekko did the exact same thing in his last game - rage quit when behind, and reconnect 3 mins later and watch until the game end.

So of course we all reported the rage quit player, however, given the fact that he did not get punished for the rage quit last game and I heard that Riot's system determines AFK/leaving the game only if the inactive duration is longer than 3 minutes, I wonder if it's possible to avoid AFK/leaving the game penalty by doing what the Ekko did: leave the game and reconnect 3 minutes later.

Apparently, he used this trick to quickly end the games that he does want to play, and he is still not caught by Riot's AFK/leaving the game detection system although we all reported him...