Ineligible Employees allowed to Defer

In 2017 and in 2018 16 employees were allowed to join the plan  and defer before meeting the eligibility requirement ( 7 in 2017 and 9 in 2018).  The  client  was told the document provider would  draft an amendment to bring these 16 into the plan. An amendment will be done for each plan year  listing  the employees by name for the respective year.  Vendor said this  will correct the problem.

Here is my question - isn't this one time amendment a "corrective amendment" and therefore had to be adopted by 10/15/2018 (for the 2017 plan year) and 10/15/2019 (for the 2018) Plan Year?

Can the plan just adopt the amendment now to correct the eligibility errors for the past two plan years. What am I missing???

Thanks 

52626, EPCRS has allowed correction by plan amendment for eligibility overinclusion for a while. See pages 116-117 of Rev. Proc. 2019-19.