even for someone moderately interested in FP, this one goes above my head and the only take-away I can get from it is "maybe use ocaml instead of haskell"

I'm reasonably versed in Haskell and my response would be that it shouldn't make that much difference to you what they've written in here. I've yet to see any code in the wild using the backpack extension.

It's more of an experiment: what if we take this beloved feature from OCaml to port it to Haskell (backpack), and see if it can replace a different beloved Haskell feature (type classes).

it's experimenting with a feature that was added to Haskell but just never caught on. it's theoretically very powerful but it's far enough from idiomatic Haskell that it sees basically zero usage in the wild as the cost for using it is that very few people can understand what you've done.