I think you're confused about how the rollback system works here.
The particular post you mentioned only has one revision. Therefore, by definition, it cannot be rolled back, because there is not a further revision to roll back. You cannot roll back a single revision because it doesn't make sense to roll back a revision to itself. Nothing would change.
The rollback-clears-spam-flags feature is to help prevent abuse. It lets spam flags be reversed when spam content was introduced into the post at a later time. In essence, the post was not spam, and then became spam, so it makes sense to be able to reverse the spam changes and keep the post in the state where it was acceptable. It is not meant to be a feature where users can go rolling back any post they want to clear out any spam flags, and if it starts getting used that way, the feature will be removed.
In a case where there's only one revision, theoretically it was never acceptable to begin with. That revision is what attracted the spam flag, and if you don't actually reverse any changes (since you can't; there's nothing to reverse), then the spam flags should not be dismissed.