It's not necessarily that they'll carry (although they often can), but merely that they win more than 50% of the time on average. Assuming you actually do belong at a higher elo, this is pretty much guaranteed to be the case, since you'll always be dragging your team up.
Some games you will have awful players you can't carry, but there will also be games where the enemy team has those awful players, so as long as you're never bad, that's 4 chances of awful on your team versus 5 chances of awful on theirs. As such, you don't actually even need to be a super skilled hard carry to climb (although that certainly makes it faster), you just need to never be a liability.
I feel like many low elo players who rage about elo hell end up feeling like they have to 1v9 hard carry, which makes them do dumb things and end up being that liability.