Why shouldn't that work? Dropbox and SMB both just need to read/write/watch files like any normal process. I do the same thing at home with Syncthing/SMB and it works fine.

From a theoretical perspective, sure, but the product Dropbox sells is either a website or an app each end user gets that adds local sync and some other useful sharing features. They certainly don’t intend it as a centralized system that people expose over the network in a bespoke way, and I can fully sympathize with why they’d tell this user that’s not something they want to support.