It is correct. You can’t euphemize a toxic behavior just because it happened in a different game mode. A toxic behavior is a toxic behavior, no matter the mode, period.
This post doesn’t say anything about voluntarily boycotting a game mode repeatedly, nor making toxic comments before leaving (no matter when it happens).
This quote was most likely regarding people having an IRL emergency at the start of a match, which is also ok in Competitive (since the match is simply canceled then).
I will look for the quote regarding disruptive gameplay Jeff Kaplan wrote and linked it to you, as well, if you want this to be that kind of “contest”.
Again, this post doesn’t say anything about the toxicity most leavers spread before leaving, nor the backfills issues.
It’s part of the leavers issue as leaving a match for other reasons than IRL emergency is inherently toxic in a game (and ruin the game experience to others). Hence, why most games punish those behaviors no matter if the match is ranked or not.
I hope you get the difference now.