Acceleration is bringing forward Completion Date to meet an earlier date. The Contractor can instruct a quote but then can not make their own assessment of the Subcontractor acceleration quote - they take it or leave it.
Here it sounds like they are instructing to speed up to bring forward your planned Completion to meet the original Completion Date so would be a CE rather than acceleration. The fact they have said it is a CE means obviously they agree you running late is not your liability.