Unless those other employees never worked more than 1,000 hours in a year, I don't see how the plan doesn't fail coverage each and every year. Remember, they may have waived participation, but they are still counted in 410(b) coverage testing as eligible, not benefiting.
Do the owners make 401(k) contributions? If so, how can the ADP test pass any given year?
[side note: did they irrevocably waive participation, or just chose not to make deferrals? If the former, something doesn't seem right. ]