It is more complex than this. The responsibility falls on the one handling the data, but then each other entity who got this data becomes also a data processor, and should be asked to delete the data. In practice it means you should request deletion to every instance who got your content through federation. This is because the instance you use has no control over what others can do, including ignoring the deletion request. I remember Dessalines mentioned that the best way is to edit all the comments. Either way, we should work on developing proper privacy policies and work together with fedi developers to provide a bulletproof privacy experience.