For sure it is. I think the big thing that makes it look any different there is just the caliber of performers. I think they're absolutely playing in the space you call out in your earlier example with our two estranged brothers, it's just that the boundaries of that space are much bigger, due to their extensive storytelling/improv experience, and established performance chemistry, plus a lot of OOC work/review they talk about pretty openly in their associated media. The all know the themes/arc that Suvi is trying to hit, they know the kinds of themes that Ame is playing with, and they have a lot of practice onboarding even quite subtle suggestions from their fellow performers and rolling with them.
I would say they're definitely doing OC play, they're just doing it very performatively and very well. One thing I think you can take from WBN (and really Brennan's style in general) as a technique to support that kind of play is how all the players relate to the rules. They only go and look at them when they want input, or they want flavor. The mechanics are one of several paintbrushes, and they only pick them up and bind themselves to them when they feel that color will help the story they're trying to tell.
Edit: I just thought of a really clear example of that from later in the podcast. Lou is pointedly putting off selecting a subclass for Eursulon, until he feels a sufficiently appropriate moment for swearing an oath and/or invoking new mechanics occurs.