I updated some metadata today, and was surprised to see that the timestamp for the sent email was rather old. Turns out, it corresponded to the time of the previous comment rather than my changes (adding me to CC and adding external buglinks).

From the email headers:

Received: by 2002:a4a:7009:0:0:0:0:0 with SMTP id r9csp2190440ooc;
        Thu, 18 Jul 2019 05:12:18 -0700 (PDT)
Date: Fri, 12 Jul 2019 15:56:06 +0000

So, the date doesn't come out of thin air, but it still seems like reporting the date of the last change (as e.g. shown if you click on the modification "History" in the bug) seems more sensible.