This is addressed in Treas. Reg §1.401(k)-3(e)(4)(ii). Since the plan is terminating due to a merger, it does not have to provide the employees with the Supplemental Notice. However, the plan sponsor is required to fund the accrued benefits through the date of termination, and the plan must satisfy ADP/ACP and top-heavy testing for the year.
Did the liability for funding the accrued benefits remain with the seller under the purchase agreement? In any event, the legally responsible party must fund the accrued benefits. Failure to do so could disqualify the plan, and bring legal action by participants and the DOL to recover the liabilities.