Brian Carpenter writes:
I am pretty sure Pipermail is a wrapper and not an inherit part of MM2.
As I understand it, Pipermail was originally a third-party add-on called HyperMail. It was sufficiently capable, and the author was amenable, so it was bundled with the Mailman 2 distribution. It can be disabled easily, and it's not hard to get Mailman 2 to use an alternative archiver if you have one (or more). However, the source of Pipermail is contained within the Mailman 2 source tree, and the configuration of Pipermail is integrated (as much as that term makes sense for a series of Python assignment statements) into Mailman 2 configuration (Defaults.py and mm_cfg.py), so it's more integrated into Mailman 2 than HyperKitty is to Mailman 3.
Thing is, there's no particular reason ever for an archiver to be integrated, with the single exception of determining the archived message's URL so it can be added to the message being distributed. And Mailman 3 shows how that can be done without knowing anything else about the archiver. So the question of whether it is a wrapper (all archivers are just wrappers) or not (containment of source code and configuration files) is kinda moot.