Just in case, the existence of a collective bargaining unit is not the only relevant condition. See IRC 410(b)(3)(A), and note especially the phrase beginning with "if":
(3) Exclusion of certain employees For purposes of this subsection, there shall be excluded from consideration—
(A) employees who are included in a unit of employees covered by an agreement which the Secretary of Labor finds to be a collective bargaining agreement between employee representatives and one or more employers, if there is evidence that retirement benefits were the subject of good faith bargaining between such employee representatives and such employer or employers,...