Since the business of the decedent was his property, upon his death the business is part of his estate which is to be administered by the executor or administrator of the estate. The executor can be the plan fiduciary or can appoint a fiduciary/PA to wind up the plan. Otherwise the plan becomes an orphan plan whis is not a preferred outcome since the owner's estate wants the benefits under the plan so that distributions can be made to the beneficaries. In order to pay distributions someone must become the plan fiducary under ERISA with authority to distribute plan assets.
The fact that the party who winds up the plan and authorized distributions did not accept fiduciary responsibility is meaningless. Under ERISA anyone performing a fiduciaty function such as distribution of plan assets is a fiduciary under ERISA, regardless of a denial of acting in a fiduciary capacity under the plan.
In other words anyone performing a fiduciary duty under ERISA is a fiduciary regardless of whether they accept no fiduciary responsibility in any written document they sign.