The way you describe what happened, that would be the only explanation.
On the other hand, I think it's also possible that he dived you with his shadow and shurikens and you responded with an auto-attack, revealing yourself and letting him ult you. If you think someone can already see you, you might just respond. Even if you don't respond, if you're not quite careful... auto-attacks will -auto- attack enemies in range unless you actively stop them (the 's' key or using movements to override your champion's auto-action). Accidentally revealing myself certainly got me killed a bunch of times as Akali until I learned what I was doing wrong and started being very careful to never be stationary under shroud (especially a shroud with an enemy in it with you, obviously, but also you need to be moving or 's' key stopped if an enemy that can burst you down is in gap close range, just in case.)
It's also possible he just put out his AoE damage - doubled from his living shadow doing shurikens and slash, and that that was enough to kill you without seeing you. Only his ult is a targeted spell. This is one of several reasons that Zed vs Akali is a terrible match for Akali to find herself in.
If you're very sure this did -not- happen, and you're quite sure you were -ulted- and not AoE'd to death, and you didn't report the player already in the post-game lobby, then you should probably file a support ticket explaining that you were hit with a targeted spell by a player that had no legitimate way to have vision of you. Also his claim to have vision of you that his teammates didn't have is effectively him stating that he was cheating (which may or may not be true, but if someone -says- that they are cheating, and -seems- to be cheating, that seems worth reporting).
As for the other issue, while being behind certainly sucks, you won't get ahead by farming a jungle that the enemy has control of (even if they are controlling it with legitimate wards and not vision hacks, it's still the same problem whether there was cheating or not). You do get experience just from being near dying minions though; just hanging out with your team while somebody else wave-clears will bring your level up -- slowly, but relatively safely. And you might even get the chance for a clean-up kill even if you're behind.