Component teams are a reality and they do exist for specific purposes. In the programs I worked on, there were some component teams that worked on specific layers of architecture such as mid-tier validations and generating the PDFs with a click to mirror the screens.
They were formed because the intended solutions at these layers required enormous explorations of different tools, open sources, design approaches, etc before these layers could cater to vertical slicing of the application to produce an increment. So essentially these teams had SME’s, Architects, or senior engineers’ part of it for a focused effort on a layered solution which otherwise was not possible to achieve as part of the feature team who primarily focus on the functional increment.