These projects seem closely related. Someone from the office-ui-fabric team made the following comment in August 2019:
We're actually in the middle of working with MSFT marketing team to somehow change this to not be branded as Office only. We are trying to become the primary way to create React (web) applications with the Fluent design language.
https://github.com/OfficeDev/office-ui-fabric-react/issues/10251#issuecomment-524402520
But the flient-ui-react website states:
Fluent UI React is being built as an exemplar of the Fluent UI design language, component specifications, and utilities.
https://microsoft.github.io/fluent-ui-react
Is fluent-ui-react simply a lighter weight alternative, an eventual replacement or something else?
https://microsoft.github.io/fluent-ui-react has been archived in the last few days, so I guess Microsoft answered this question :)