That's definitely what they meant, that it's easier to create an original plot involving two (or more) central characters when each character's background, personality, and physical appearance have already been created.

In Arcane, the writers had free reign in writing story lines as long as they stuck to the basics of the lore (Vi and Jinx being sisters; Vi acquiring her gauntlets; Jinx going "crazy"; both being at-odds with each other), including the lore of other characters added into the show (Jayce developing hextech and becoming a leader; Viktor eventually ending up as he does in the game; Caitlyn becoming Sheriff of Piltover; Ekko developing his 'powers'; etc.).

In The Last of Us, the video game is the story line. It tells the entire thing from start to finish. The story of Joel and Ellie in the first season must start with Joel meeting Ellie (Point A) and end with the decision Joel makes at the end (Point B). There is no room to maneuver there for the writers, other than how the story gets from Point A to Point B (and they've cleverly added in some flashback material to flesh out certain characters and some key questions surrounding the infection).

While Arcane is certainly exceptional in its creativity, its aesthetic, its marriage of action and dramatic tension, and its engaging cast of characters, it certainly had an easier time offering something new for viewers compared to The Last of Us.

With that said, I think both are exceptional.