I am not a lawyer but section 5 states that this only a suggestion.
Sec. 5. This order does not require an executive agency to use a project labor agreement on any construction project, nor does it preclude the use of a project labor agreement in circumstances not covered by this order, including leasehold arrangements and projects receiving Federal financial assistance. This order also does not require contractors or subcontractors to enter into a project labor agreement with any particular labor organization.