Turns out I made some unnecessary "work" for myself when I tried to add support for command history to inf-mongo. As Mickey over at Mastering Emacs points out in a blog post, comint mode already comes with M-n and M-p mapped to comint-next-input and comint-previous-input. And of course they work in inf-mongo right out of the box. I still prefer using M-up and M-down, plus I learned a bit about sparse key maps and general interaction with comint-mode. So from that perspective, no time was wasted although it wasn't strictly necessary to put in the work.
But with Emacs being the box of wonders it is, it's still fun to learn about features and new ways of doing things even after using it for a couple of decades.
There are a more gems hidden in Mickey's blog post so if you're using anything that is based on comint, I would really recommend reading it.