Maybe the “soft tips impart more spin“ argument could work like this: Because softer tips hold chalk better, and therefore miscue less often, you can confidently get further out to the side of the cue ball, and therefore get more spin.
That is plausible, especially if the people with that belief don't chalk carefully and often enough. In my experience, the hard playing tips I have used hold chalk fine. Phenolic tips are a different story.