Ok, lesson learned, never argue with Niko.

From this info from Nordeus it is obvious that the ball possession bonus works like Niko described:
Live Match Supporters - Top Eleven be a Football Manager

It clearly say that if you have x% in ball possession bonus, you will get x% more in possession.

However, this function can of course not be used with the team bonuses of attack, defense and condition up to 10%. Like, you cannot get 10% "more" attack, 10% more defense. I would still bet that these bonuses increase the ability of the players. I think these bonuses work parallel with the ball possession bonus from watching a game, crowd and supporters. Or? Anyone having any other idea?