That's more accurate, Soul was probably rendered at 2k, as most of their movies.

If a shot doesn't look good enough when upscaled, they re-render in 4k.

Actually, they used to upscale most of the movies using Nuke, but recently, started using deep learning for that. At Siggraph 2020, they gave a talk on this topic: https://s2020.siggraph.org/presentation/?id=gensub_443&sess=...