WRT whether the creation of a backlink on a work item should be update the modification time of that work item could be argued either way.  An argument in favor of doing so is the one you made, i.e. a change shows up in the history, so the modification time should be updated.   An argument against doing so is that the user never "saved" a change to that work item, so they would not expect the modification time to be updated.  Unless this was a critical bug, it is unlikely that the development team would make this change, since there could be a lot of customer scripts that depend on the existing semantics.