"Cool story bro" is borderline toxic, since it is thick with sarcasm and antagonism. Given the short context of the report (and without actually seeing the chat log myself), I would have to guess that the reportee (defendant?) was being antagonist towards the reporter (plaintiff?) through his continued responses with this comment. It is a behavior that should be address and, were I on the Tribunal panel that received this case (and based solely off the little bit of information you were understandably able to provide here) I would vote Punish.
However, this is far from an extreme case and I would not agree that it is "toxic as fuck." If I had a say in the level of punishment applied (which the old Tribunal lacked), I would suggest a warning, or depending on the chat log I might would suggest a minor chat ban or very limited ranked restriction (24-48 hours max) based on this incident in isolation.
Severe instances of toxicity in my experience occur when the offender is gassed up and egged on by other teammates, or even opponents in some cases. A comment in champion select such as: "F*** you guys, imma dbl jung and fd" would incite anger in teammates, and opponents would probably comment on the double smite comp in All Chat before leaving the fountain. Then an All Chat conversation starts up, usually smashing the offender for his toxic behavior, and then the feeding begins. This is followed by cursing and likely an AFK or two. What has happened here is every member of the offender's team (himself included) becomes dejected and all possible enjoyment of the game itself has gone the way of the dodo.
Another instance I have actually experienced firsthand was a full-on troll. He called a role in Champion Select, was granted that role, and after an earlier pick chose a champion and role he then decides he'll go mid instead. In an effort to avoid conflict and silence the troll, the earlier pick decides with great restraint "whatever, I'll jungle." Cool, right? Well it was, until the troll chooses no first items, says in team chat, "I always win if I give first blood," goes to the mid lane river, and says in All Chat, "come mid for first blood." He delivers said first blood then leaves the match. Easily the most toxic experience I've ever had in this game.
"Cool story bro" certainly doesn't rise to the level of either of those cases, and I think that is your main point, AKG. That doesn't mean that it is justifiable to reply with this comment repeatedly in an act of passive aggression, though.