IIRC Dropbox was originally created using librsync

Dropbox has modified it

https://github.com/dropbox/librsync

This is why I prefer open source software. I can modify it

One person can use librsync to create a Dropbox company. Another person can use librsync for noncommercial purposes, e.g., to transfer and sync their own files

Either way, it's librsync

I mean... I, for my own needs, which are rather simple, can replace Dropbox with rsync. That's one thing. But yes, it's an entirely different thing to consider you don't need, or worse, could own, such business, on the simple premise you don't need it at your own level. That would be madness to mistake one for the other.