To the library authors's credit, I think creating a general DnD library that can address most use cases is tough.
I would love to see your 100-200 lines if you are willing to share. Not asking you to ship it and support it and end up the N+1'th unsupported DND library though ;)
My implementation doesn't generalize at all really and I only need to support desktop browsers. If I ever own source this app I'll let you know. But basically I have one component listening to mouse events and firing callbacks with the rectangle the user has dragged (mouse down point to most recent mouse point), and with that my state manager updates things accordingly and drops upon mouse release.
I wrote my own as well, not in 100-200 lines granted. Dependency free ~300-400sloc using basic mouse events and element calculations were enough to implement advanced behaviors such as snapping, collision detection, free movement, packing, swapping, containerization. I had lots of freedom on how it behaves and was able to tweak it to what I wanted, something probably impossible with general dnd libraries. It works the same in most modern browsers I've tested, since it's only using DOM apis for basic events and style modification.
Libraries are great if you need established algorithms and data structures. Building out custom UX interactions, presumably like interacting with DOM objects, DIY was easier since there's actually very little work to do.