The IRS defines superseded returns in IRM 21.6.7.4.10 (10-01-2021) that does say individual tax returns superseded before the due date of the return.
Chapter 21.6 of the IRM is Individual Tax Returns - not business returns.
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An amended (Form 1040-X) or corrected (duplicate) return filed on or before the due date or the extended due date is a superseding return.
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Correspondence postmarked on or before the due date or extended due date, requesting changes to tax returns, is processed as superseding information.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
