Yeah, the image (not mine, but the best I found quickly) kinda shows a rebase+merge as the third image. As the other commenter mentioned, the new commit in the second image is the merge commit that would include any conflict resolutions.

why would rebasing a feature branch change main?

the image does not update the feature branch. It merges the featurebranch into main with a regular old merge-commit on the main branch.

The only difference between a *rebase-merge and a rebase is whether main is reset to it or not. If you kept the main branch label on D and added a feature branch label on G', that would be what @andrew@lemmy.stuart.fun meant.