Before I go down this rabbit hole: was there any particular reason (back in the day) that Pipermail was favoured (and implemented) over MHonArc.
Mailman was initially implemented by John Viega in the mid 1990s to manage a mailing list for fans of the Dave Mathews Band. I don't know why pipermail was chosen, but MHonArc was fairly new at that time and pipermail was probably more mature.
I doubt that Hypermail (the original code) was more mature than MHonArc, but it was written in Python and so considered more appropriate for bundling with Mailman. ... As far as I can recall, Pipermail never been especially recommended over 3rd party solutions like MHonArc or external services like mailarchive.com, it's just easy to use because it's guaranteed to be there, we provide support for it, and it turned out to be "good enough" for an awful lot of lists.
Many thanks Steve and Mark.