Assumptions are Risks
In other words, "we assume team X will do this, and team Y will do that. If they don't, we're not responsible when our plan fails."
How should a project manager respond to such a list?
Assumptions are risks. Replace assumptions with specific deliverables or activities to reduce risk. There should be no assumptions in a project schedule, only deliverables and their associated activities.
If there is any concern about whether team X or Y will deliver, add it to the risk list and manage it according to your risk management plan.