Schedule MEP - Who Must File

The General Instructions for Schedule MEP states "All MEPs must complete Parts I and II to indicate the specific type of plan or arrangement..."  Under Who Must File it states "Schedule MEP (Form 5500) must be attached to a Form 5500 or Form 5500-SF filed for a pension plan that checks the “multiple-employer plan” box on Part I of Form 5500 or Form 5500-SF..."

My MEP experience is limited to closed MEP's so I'm looking for some clarity.  Are participating employers in an open MEP required to file Schedule MEP with their 2023 Form 5500-SF's?  The MEP administrator is saying no, because they have never checked the multiple-employer plan box on Part I of the Form 5500-SF.