I think the tabloid is looking for any reason to generate drama. People are looking for conflict when there is none.
When Harry and Meghan left, their calendar is no longer dictated by the Palace. They are under no obligation to coordinate anything with the Palace. This is a foreseeable situation. If the courtiers did not negotiate this point during that summit, then they have failed in their duty and they have only themselves to blame.
Charles picked his coronation date on Archie's birthday. If Harry and Meghan stayed as working royals, maybe he wouldn't have chosen that date, but since they are not working royals, that was obviously not a consideration. (Also, who cares about a 4 yr old's birthday? Even the kid won't remember it.)
If you don't want Harry to write a tell-all book, then keep him in the fold. In hindsight, I think people should realize that keeping Harry in the family would have been better. Whatever price he asked for is worth it considering the fall out from Oprah's interview, the book, etc.
Considering that he wanted out of the British Royal Family and had indicated so for awhile, I'm not sure that "keeping Harry in the family" would have even been possible.
Also I do agree with those that pointed out that Charles did not "choose his coronation date." That decision would have been made equally with the British Government. All involved in the planning would be taking many aspects into consideration: weather, the schedules of elected officials and diplomats, and other major events happening in the nation and other parts of the world.