Those were the quick view / quick boot festures that old Dell and the likes had. Usually booting into some bars ones productivity apps.
I did see a video on YT that did a deep dive into this. I beleive you are correct they were originally stripped down Linux or other custom OSes but then eventually switching to UEFI apps before falling okay of favor.
I think this might be the video you're talking about, in case anyone is also interested in a brief history of quick boot operating systems and UEFI: https://www.youtube.com/watch?v=ssob-7sGVWs
I did see a video on YT that did a deep dive into this. I beleive you are correct they were originally stripped down Linux or other custom OSes but then eventually switching to UEFI apps before falling okay of favor.