welcome - well also emissions e.g. as output flows can be uncertain, and openLCA calculates the overall emissions for your system under study, and obtains a final aggregated result for the emission flows which is also uncertain. This speaks for specifying uncertainty also for output flows.
The only point is that some emissions depend directly on input flows (CO2 emissions from driving a car on the fuel consumed, e.g.) which is typically not reflected in LCA datasets, and thus these flows are simulated indepenently, which overestimates the uncertainty for them. Since this dependency applies only for some cases, I would say it is still ok to consider that output flows are uncertain.
Quantitative references: the entire process data set is scaled to the amount of the qr, so this one amount is not uncertain because it is decided by the modeler or the dataset creator.