AD Carry main here. Still don't see a problem with this.
Jhin is not a duelist. What's more, he's built for ranges longer than this. He explicitly has two abilities with 3000+ range. Rift Walk goes 500. I can't tell from the video, but I'd bet Rift Walk is stacked at least twice before the fight starts. Then Sneaky walks up next to the brush that he's in with his E and trinket both down so he can't check brush.
So why is the duel close? Because he took the actual worst possible fight. He eliminated every single advantage his character has, gave every possible advantage to the Kassadin, and EVEN THEN, still won because of a gold lead. That's why the duel is close. You fought a multi-stack Riftwalk Kassadin by letting him engage on you. You've been playing this game for ten years and you can't answer why the duel was close? I know you're better than that Sneaky. I know you're smarter than that.
You're absolutely right that if the roles were reversed you'd get slapped. But you'd also know not to face check a Kassadin with Rift Walk stacks.
Meanwhile, in the real game of League of Legends, we aren't playing "Jhin vs. Kassadin 1v1." There's a reason Jhin isn't played as a Top Laner while Lucian, Vayne, and Tristana are. Jhin isn't built for 1v1s. He's built for team fights and supporting his team from off screen where he's literally untouchable. Willfully forsaking every single advantage your character has and then going "DAE RIOT BALANCE?!" makes you foolish.
For reference, if you're Lucian and you intend to take on this matchup, you would have gotten a Maw. Maybe even Scimitar on top. Life Steal regardless with Ruined King. The fight would be SUPER not close. Lucian would come out above half HP even with face checking the Kassadin and letting him open with Rift Walk.
Let's circle back to a real game of League of Legends. Exactly what output do you expect Kassadin to have in a teamfight here? He's going to get instantly stunned by a Skarner, chain stunned by Jhin, and die in literally four auto-attacks as shown here. You know, the real game of League of Legends where you coordinate with your team, use range and positioning advantages, and have more than half your abilities available to you. Super not close. I could win the game in Sneaky's shoes in a teamfight and I'm garbage.
It honestly feels like every "DAE AD CARRIES SUCK?" clip I see here is of people who actively int themselves into enemy assassins and wonder why they can't just take their hands off their mice and win. I really don't get it.