The above paragraph suggests that all changes are done in the Grab Mode instead of the Internal Edit Mode. It isn't intuitive to change the mode from G to E to edit the patch changes in view of the documentation and several users ran into this problem:
https://discourse.ardour.org/t/how-do-i-modify-delete-a-patch-change/88413
https://discourse.ardour.org/t/g-e-mousemode-colors/89766
The documentation is clarified that while insertion happens in G mode, modification hapens in E mode.