I also believe it’s about the type of movement rather than the speed. Workouts learns about your run and your walk and optimizes tracking, so if you happen to be a fast walker or a slow runner/jogger, it’ll pick up on that.
It’s really your preference as to what you want to see in record and shared activity, too. My runs involve some intermittent walking, not enough to warrant switching workout types or IMO enough to confuse tracking, and it’s my preference to see those as runs in my history rather than walks with intermittent running, but either would work.