Most ranters don't think they're ranting, so they aren't going to have the self-awareness to click on the rant board and admit that their thread is stupid. It can only be another GD/Meme board for ironic "rants" and meta commentary.
Even if it worked as intended, the idea of a containment board has pretty much already been disproven on other websites (reddit, 4chan, etc.) as the existence of the board still makes it a part of the whole site's culture, creates a space for those bad attitudes to breed, and the users go to other boards and spread that same mentality. /pol/ as a containment board made all of 4chan get steadily worse in the long run even if turning it off made 4chan immediately worse in the short run.
Riot should address problems with the boards ("discussion mode" is wothless for actually having a discussion because reddit was never designed for discussion, search is so feature-limited that it is functionally useless) and their reddit presence (all discussion in a single sub whether it's esports, patch notes, twitch e-celebs, fanart, memes, or gameplay discussion so only around 5 conversations can happen at a single time and if you aren't one of the first 10 commenters your voice is completely lost) and this fails to address anything.
The post at the top of "RANT" about rules for the board will also end up being pointless. The "RANT" board was created for users to put up the worst posts they can think of, there's no way they'll suddenly develop the self control to not throw in hate speech, personal attacks, and death threats when they would already do that on posts they actually thought were good.
This is basically the "Prisoner's island" of posting except there's still a bridge from the toxic island to where all the "good" people are supposed to be.