I note that 401(k)s do not backdate documents all the time. That is a scary statement.
So, while I cannot address your question, I will address your statements.
Only under very limited circumstances can a plan sponsor retroactively adopt 401(k) provisions, and this is newly available under SECURE 2.0. Retroactive 401(k)s are permitted beginning with the 2023 plan year for Schedule C filers who have no common-law employees. This is permitted with an unextended filing deadline in 2024.
SECURE 1.0 introduced retroactive adoption for employer contribution only plans effective beginning with the 2020 plan year. While the funding deadline for retroactively adopted DC plans is the due date of the entity tax return for the applicable plan year, which includes extension, the extended funding deadline for retroactively adopted DB plans is 8 1/2 months following the end of the plan year.
Be careful about quoting calendar dates. Not all plans/plan sponsors are on a calendar year cycle, and different entity types have different filing deadlines.
Prior to the enactment of the SECURE Acts, all qualified retirement plans were required to be adopted by the last day of the applicable plan year, and 401(k) was a prospective provision within the plan document (and still is, with the exception noted above).
Retroactive adoption and backdating are very different. Retroactive adoption is permitted under specified circumstances. Backdating is always fraud.