I don't think you can equate someone anonymously flagging posts with a 'toxic' forum. It's not active enough to have an atmosphere!
The flagging does essentially nothing. Doesn't remove the post, doesn't stop the poster posting, doesn't even give the poster a notification. On a mobile browser it is barely noticeable.
There has to be more to it. I suspect that person, wherever it was, tried to ask a question and was either ignored, fell foul of the rules and felt upset, or had a joke at their expense that they didn't get - then they went somewhere else to badmouth this forum.
If we're talking improvements, how about changing the one-click flag option to a more developed 'report' option where you click once to open a menu, select a reason, type something and submit. The more clicks, the less likely a joker is to abuse the system, and because a user has to submit it their username is probably attached to it instead of the current seemingly-anonymous method.