That blog posting is from 2012, so I think it's surprising that its author clearly hadn't read RFC 4409, published in 2006.

The first paragraph about message modification is: "Sites MAY modify submissions to ensure compliance with standards and site policy. This section describes a number of such modifications that are often considered useful." From is typically modified to comply with SPF and/or DKIM.

RFC 4409 is obsoleted by RFC 6409 (Nov. 2011), which is the one I refer to. The sections therein on header rewriting do not allow the modifications that Google makes to the From: header, which can, as I point out, lead to mismatches of personal names and email addresses and falsification of sender identity, with all the easily foreseeable damage that may result.

6409 contains the same wording: "MAY modify submissions to ensure compliance with standards and site policy" and gives examples.

The sections on header rewriting are essentially examples of what the document describes as "modifications that are often considered useful", they're not an exhaustive list. The only MUST NOT rule that concerns email address rewriting is about destination addresses.

Is it your opinion that a reasonable interpretation of 6409 is that a submission agent may falsify the sender of an email? Or create false associations between personal names and email addresses?

No RFC rule prevents that, and as far as I know no RFCs even try to define what constitutes a true or false association between a personal name and an email address. But RFC 6409 gives site policy unrestricted permission to allow or deny that.

(Well, almost unrestricted. The results have to be syntactically valid.)

I don't think your interpretation of the RFCs is natural or remotely reasonable; in fact it leads to absurd consequences. We'll have to leave it at that. I stand by everything in the article.