From Chauhan, V. M., Zhang, H., Dalby, P. A., & Aylott, J. W. (2020). Advancements in the co-formulation of biologic therapeutics. Journal of Controlled Release, 327, 397-405.:
Co-formulation can be considered as the purist view of a combination therapy and can be described as consisting of more than one drug substance in a single formulation, with the intention of delivering multiple therapeutic agents at the same time for maximum therapeutic benefit
Co-formulated in the context of a therapeutic drug/treatment means that two or more components are part of the same combined treatment, for example existing in the same pill or solution for injection. In this form, they are not separable.
It's not clear to me that this is specifically what the chart you linked means, though, rather they are saying that some treatment options have not been compared independent of the others, but were always given together within trials. That might be better described as "co-administered".
From the context of the flow chart, it is not immediately clear to me which specific combinations they are referring to.