Establishing a PS only plan after plan year ends

We have a new client who wants to establish a Profit Sharing Plan for 2021.  The client (12/31 FYE)  tells us that they already filed their 2021 taxes (and did not put their return on extension.)

My gut response is 'it is no late' since filing deadline as passed.

Now what is the client filed an extension (deadline now 10/15), but still has already filed their corporate return  last week.  Does this change my response?  I am thinking so, since the corporate return was extended.  Do you agree?

If anyone can provide documentation to support or refute my answers, please provide.  Thanks.

Too late, needed to be done by the original filing date if no extension.

If the Client has a valid extension and did not file by the original deadline I believe that would change the response as the could can file an amended return. Or it may be a replacement return in this case as I'm not a CPA and think there might be a subtle technical difference that eludes me.