All observations are interpreted in the context of the expectations of some theory, the issue is whether the observations are needed to help confirm the theory or if the theory is already on a strong enough foundation that you can use it to draw other inferences. I'd say the theories of nucleosynthesis and general relativity are in the latter category, so it is acceptable to consider light element abundances to be observational constraints on baryonic densities. There are always a few details to hammer out, and big surprises are also possible, but what else is new.