This test fails for every metabolite that is not involved in at least one interaction (see doi:10.1371/journal.pone.0263057). This interaction can be, for example, a metabolic conversion or a binding to a receptor.
The connectivity helps network analysis approaches that use WikiPathways (see for example doi:10.1039/D3DD00069A).
Basically, when there is no interaction which involves the metabolite, it will be highlighted. Possible reasons are that it is not linked at all, linked via a graphical line, or the only other participant in the interaction does not have a recognized identifier.
Connect the metabolite DataNode with an Interaction.