I don't see how this is true. If you're building slack into your cycle there should be regular investment time available that a pure flow-based system wouldn't have.
If there's no slack in your sprint schedule, well there's your problem.
In my experience, Scrum's work-unit factorization encourages thinking of developers as spherical cows that need no personal development, with slack as a concession to reality that patches the damage Scrum itself causes. And Slack is generally considered to be for technical investment, though, not personal investment.
A good manager could think past that, for sure, but I've never encountered a good manager that felt Scrum was good or necessary for their teams.
If there's no slack in your sprint schedule, well there's your problem.