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.
If there's no slack in your sprint schedule, well there's your problem.
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.
Then management comes over and asks why you're slacking off.