This is an accurate summation of how ADC agency compared to Mid or other lane's agency. ADCs, even at their strongest point, have extremely limited means by which to carry the game solo. Its insanely frustrating as an ADC player because you either have to play perfectly or your team does. There are games where I'm carrying my ass off, I'm doing well, but my mid is really stupid and has fed the ever loving hell out of the jungler and the assassin. There's next to nothing I can do unless I constantly monitor the assassin and the enemy jungler; I always have to be keeping track of their CDs or praying that they'll blow them on my team. And the worst part is that if I don't group, I'm going to get flamed but if I do group, I'm going to be irrelevant. There's no winning for me.
Then there are other games where I just fart around bot, I don't get ahead but I don't feed, but my mid and jungle are doing great. And I'm just there; I could be anyone else playing the ADC that game and it wouldn't matter. And that feels bad in its own right. Because I'm not important, I don't add or have any impact with the game because my mid and jungle honestly don't need me. They can siege and teamfight and out put enough damage without me. I'm just a free meal for their behind laner because even a feeding assassin or mage can 1-shot me.
Even if I play absolutely perfectly, I build defensively(opting into defensive boots instead of greaves, getting PD and a GA early, taking weaker rune choices for better survivability...), there's still a chance I will lose because my jungler or mid are behind and they can't defend or they're tilted and just running into the enemy team essentially to give away free kills. Even if I survive, its not like I can solo defend with how paper weak the towers are either...
I get ADCs have supposedly amazing late games. And some of them do. But at the same time, they have nerfed a lot of ADC late games without really buffing their early to mid game or giving them access to extra agency. Carrying as an ADC is literally the hardest thing to do.