That's partly true.
But this year in particular, it's sort of extreme. You mess up once, you are behind like 3 levels. You don't mess up once, but don't actually manage to get ahead, then compared to the rest of the game, you are still behind 1-2 levels.
And I mean, it's been this way since as long as I can remember, but if you mess up more than once, you're probably better off afk so you don't feed the enemy team anywhere you go.
It's not really an exaggeration. The jungle changes this year are pretty crippling, and A LOT of it has to do with how much power Riot wanted to put into dragons and the possibility of a second rift herald. They took all of the power from the jungle, and put it into "4 man bot asap and take drag hurr durr"
They're on the right track in nerfing dragons in the upcoming patch (except cloud, they're buffing that one, and honestly, that's fair) but you can legitimately level faster just joining a solo lane, unless you get ahead. Going even, you're still like 1.5 levels behind the solo lanes, and often about a level behind bot.
I get that failing within your role is supposed to be punishing. I don't mind falling horribly behind if I'm playing like shit (read: no I'm not, I'm tilted af...but I know why I'm behind...) but when nothing has gone wrong yet and I'm still level 4 to everyone else's level 6, there's an issue.