You seem to have a few misconceptions on how the system works. It's not a "I'm going to judge and punish 1 player in that game" system, but instead, it's a system that punishes based on consistency x severity. Meaning you can get punished for 1 game of extreme toxicity or many games of mild toxicity.
To put it in order words, think of the system like a health bar (or tension meter) in music games. When you hit the note at an off beat, your tension meter increases. When you completely miss a note, the meter increases by alot. When you hit the note at good timings, it reduces the meter.
Riot's behavior system works the same way, as in we all have a toxicity meter attached to our accounts. When players are mildly toxic, it goes up a tad. When people use hate speech, it pretty much fills up the meter. of course, clean games reduce the meter.
It is possible for playerA to get punished and playerB not to, even though playerB was waaayyyyy more toxic . Let's say playerA started the game with the toxicity meter at 95% and playerB's at 35%. Ingame, playerB said some pretty nasty shit and playerA responds with some minor shit as well. After the game, playerA gains 10% toxicity and playerB gains 50%. Now playerA is at 105% toxicity and gets punished, whereas playerB is at 85%, and not punished in that specific game, even though playerB was found to be 5x more toxic than playerA in that game.
The logs of other people don't matter because everyone has a different behavioral history. Every reported person will have the game reviewed independantly from others and the review score accumulated into their individual behavioral history.