Thanks Draper and CB - Yes, there was one non-owner participant who terminated (with a vested benefit) in a prior year (before 2021) and was paid out in 2021. So once this participant was paid out, the plan only covered the 100% owner. The non-owner terminated vested participant will not receive an employer allocation in the 401k PS for 2021.
Agree, absent clear guidance to the contrary, the "safest" approach seems to be limit the ER contribution to 6% on the DC side for the 2021 plan year.