Okay, this saying is starting to piss me off.
First, I'm an AP mage main. I "main" mid lane and support. I play an okay top, a meh adc, and a below average jungler.
I'm not going to spend time practicing my 5th role, because I only get it (If i'm lucky) every 10-15 games.
"Lol then how are you even Plat?" Because I'm good at my other roles. I'm a Plat level mid-support, I'm probably a silver level jungler. When I get that position more often than not I'm against a jungler main.
It's seriously stupid to expect every player to be godlike at every role. There's a reason people call out their preferences. Yes, you won't always get your role and you should be knowledgeable of the basics for every role.
Even pro players usually only play their one role, but they always get it because they have name recognition.