I found my answer in some presentation material that Sal Tripodi provided in a webinar that he did, I think for FT William, on the SECURE Act. He provides an example of a new calendar year end plan adopted 11/1/2021 with a retroactive date to 1/1/2021 (401(k) portion effective 11/1/2021). Sal offers that the plan could not be a safe harbor plan for 2021 since the 401(k) portion is less than 3 months, regardless of whether the safe harbor is a match or nonelective.
Makes sense. The intention of the 3 months is to allow everyone the chance to make 3 months of deferrals regardless of a match or 3% non-elective. Without the 3 month requirement I guess an owner could set up the plan for 12/31 and give himself a bonus to defer from and no one else gets that opportunity.