There's an attack in your video where the banished knight makes a little backstep before coming down with an overhead swing, that's probably why that couldn't be parried.
Dude every attempted parry in that video has the knights hands clipping into the PC. There's no way he's too far away.
That's not true.
The only ambiguous parry is the one at 0:27, where he hits the shaft of the halberd with the shield but parries the second swing since he is now a little closer to the body. The rest i don't see a parry opportunity. There's an attack where he swings around to create a whirlwind but doesn't really feel like an attack, more like a prelude for something else. There's several where he back steps as the weapon is coming down which definitely couldn't be parried. The one where he follows up an overhead swing with a low attack where he scrapes the ground and then goes for an upward swing while moving his body sideways is not something i personally would assume could be parried.
Keep in mind i'm not saying there aren't situations where it's a bit ambigous whether you can parry an attack or not but usually if it's a matter of using parry because rolling doesn't seem as effective you generally only have to focus on those specific attacks you are sure can be parried. I don't know if EVERY swing the dual wielding banished knight makes can be parried, but in reality what difference does it make, you only need the one:
When i tried to do this boss fight with rolling the dual wielding knight had me dick around the arena for like 10 minutes fuck that shit. One good parry and bam, he is gone.
One thing to keep in mind is that unlike previous Souls games, Elden Ring allows you to use the best parries on 100% medium shields. This makes things even easier since you can mix up parry with blocking, meaning parry is not as risky as it used to be which further stresses that it has now been "upgraded" to being a standard tool, which also means you only have to use it if you feel you need it.
The dual wielding knight you fight on the bridge for instance could be done with parry also, but he is so close that sometimes it's best to rely on the great shield bounce, as i displayed recently:
Everything in this game is situational. I can see if you went through Dark Souls 1 to 3 with rolling only it can be frustrating to be forced to employ unfamiliar play styles but honestly i don't see how they could just keep the roll-fu stuff going indefinitely. In a way it's also the reason they did Sekiro in the first place.