The issue is that the observer function you pass to runtime.module is only invoked for symbols that are defined by the notebook itself, not for symbols that are imported. So, you need to give the cell that references the imported cell a name (as @bgchen suggested) in order to make it available for embedding.
But I think we can fix this on our side so that imported symbols are observable too (i.e., the observer function is invoked for each import in addition to cells defined in the main notebook), and then you won’t need to give imported symbols an additional name for embeds.