Oh. Raise you hand if you hadn't been considering how your metabolites might be directly interacting with your proteins and that it might be really powerful to do so.
A really cool part of this method is how this can be a one pot solution. Despite how we seem to be divided into different camps between proteomics and metabolomics, basically all of today's high resolution instruments can do both really really well. In this case the authors do both great metabolite ID and proteomics on a Q Exactive Plus instrument.