That was enough of a hint for me to sniff out a more confident answer:
My suspicion is that “MiMi” is a truncation of “Mimic”, which was Mycroft’s family of TTS engines, and the listings are white-labeled hardware configurations/revisions. However, it does not explain the $25 accrued monthly coupon, nor does it explain the “Mark-II+/MiMi 15,000”, which easily surpasses even the most expensive Librem Server v2 configuration. My best guess is that the $25 accured monthly coupon is some sort of subscription required to utilize these listings.