Don't think I could disagree any harder. Requirements change. Business needs change. You want your FE to be able to evolve independently without having to wait for the BE to implement the precise logic you need.
This doesn't mean that an actual cross-functional team that contains both frontend and backend developers can't use the proposed single-purpose API to run circles around the split team.
For clarity, I don't think you need to treat your own front-end like an external consumer & think it's fine to have bespoke endpoints as needed.
What I'm skeptical of is the author's suggestion that a backend endpoint brings back presentational info like "leftBoxTitle".