Your question could be worded a little better. However what I think you meant to ask is, "Under what circumstances would a plan which covers only the substantial owner(s) of a business and their spouse(s) not be exempt from PBGC coverage under the substantial owners exemption?" This might shed some light on the question:
https://www.pbgc.gov/prac/other-guidance/insurance-coverage#substantial
If spousal attribution only applies in the case of a corporation, then in the case of a partnership, the spouse must themselves directly or indirectly own more than 10% of the profits or capital interest (since spousal attribution doesn't apply, I am not sure what it would mean to "indirectly" own an interest in a partnership, I am thinking community property maybe). In the case of a sole proprietorship, the spouse could never be a substantial owner.
If the business is a professional service employer, then they could still be exempt even if the spouse is not considered a substantial owner.