Does everyone need to sit, even zen masters, especially according to Soto zen? Or are certain highly accomplished meditators incapable of experiencing scattered thoughts?
When you don't have "scattered thoughts" whenever you are doing something, you're doing it fully immersed. Everybody has had periods of absolute involvement.
Obviously everybody has thoughts, it's just a matter of Distance from them.
As a matter of tradition and to set an example to everyone, everybody sits. I think even the founder Dōgen Zenji, sat till the very end. I'd even go so far as to say, if some people sit and some don't need to, then there's something wrong with the tradition and has some biases.
Also some accomplished masters just sit peacefully/blissfully. Let's say you have mastered a good amount of distance between yourself and thoughts, you will be able to sit peacefully. And that would be a pleasure (not the material pleasure, just bliss). Hence you would just sit naturally. I hope I made myself clear.
In Soto Zen we don't sit to accomplish any end or fulfill any need, we sit (and do the rest of our practice) because it's the realisation (i.e. making real) of our Budda Nature.