This is a good point. Scrum's sprint length inflexibility frequently means over or under-allocating time on projects, which messes up metrics (e.g., ticket completion per sprint) ... but is otherwise irrelevant to the work itself. This leads to organizations making sub-optimal choices that help with scrum, but don't help with work.
Yes from experience over the past year working in PM, the sprint and milestones are very different things. Sprints are predictable and pre-scheduled. Milestones have an estimate but may easily slip from one sprint to another.