Look, I get your point. The other guy is solo carrying, so clearly it's not impossible, right?
Except that's not the point people are making... The guy who got fed and solo carried, didn't do it because he played well. He did it because his opponent played poorly.
Obviously it's hard to conclude that naturally, but when you're trying to carry versus this guy, and outplay him so hard multiple times, but it doesn't matter because of his item lead, that's why people feel this way.
Like when I'm 3/0 and can kill the enemy 8/0 1v1, then kill him again 1v1, and continue to kill him 1v1 every chance I get, BUT between each of those 1v1s he gets 2-3 kills on my idiot allies. I'm the better player, he's got 5, 7, 11 kills up on me and I'm still killing him in 1v1s when he's on the duelist and I'm on the teamfight champ. That's where people complain that you can't solo carry. He didn't solo carry - my allies "multi-fed". I couldn't solo carry them.