It very well may have caused an immediate punishment. He may have been given a chat restriction, if he has not had a history of negative behavior. You would only see the pop-up if yours is the last report that triggers a punishment; i.e. if someone else in that same match saw the kys and reported him for it, but their report went through even a half-second after yours, it would have given them the pop-up instead of you.
And just to clarify a few things... while the report system does function based upon a percentage of negativity, the severity of the infractions is taken into account. Certain things, like hate speech or death threats, do have a zero-tolerance policy and can often result in an escalation of punishment up to a 14 day or even permanent suspension for a first offense. However, that does not mean that a player who has an extremely low report percentage will always immediately be given a permaban if they happen to type %%% one time in one match. It has been done, I've seen many cases where players have come through the boards with no prior punishments, but been given a permanent suspension for one match that uses hate speech, but that does not mean that every single case will happen that way.
Just as the system is set up to punish certain things strictly, it is also set up to punish them fairly. And that is where the "report threshold" comes into play. It's not that a player has to have a certain number of reports, so to speak, because even if a player has played thousands of games since season one, that will not necessarily offset a streak of thirty toxic games in season seven. It is true that newer accounts can end up punished more frequently because they have fewer positive behavior matches than older accounts, but just having more games won't give the older accounts a free-pass to be more toxic. What is taken into account is whether the accounts have demonstrated a pattern of consistent toxicity. When setting up the IFS, Riot set out to not punish players for having occasional slip-ups or moments of mild toxicity. It's a competitive game, and people get worked up. Riot understands this, and doesn't want to punish players too harshly, or become so strict that it removes any element of banter, or teasing interaction between players. So, if an account is reported once out of a hundred matches, and the IFS only finds one comment in the logs that is toxic, and it's not something that is considered to be extremely toxic, Riot is not going to consider it fair to immediately punish that player. No one would. So the report is applied to the player's account, and if the player continues to be negative, they will be punished. But if they return to their good behavior, then they will not be punished unless they start having more toxic matches.