I see your sticking point.
The form is asking for the topic which you are attempting to update the custom data for (which is the stock action I referred to). But unless there is an explicit topic in the event arguments, how could rules give you an option to choose from? In this case, not just topic comments can be reported, but all sorts of other content can be reported. It cannot be assumed that you can derive a topic in from any given piece of reported content. If I report a comment on an image, and that rule event is invoked, how is rules going to get a topic from that comment? It's undefined.
This is why you only have the option to use PHP code to provide the topic for which the stock custom data update action will operate on. Rules knows that it cannot know what topic to use. Therefore you must tell it.
You could put a condition on the rule that checks if the content is an object of the 'IPS\forums\Comment' class, which would ensure the rule only runs when a topic comment is reported. This will prevent errors when other types of content are reported, but you are still left with the need to provide the topic itself for the custom data update action. You need to return the topic from your PHP snippet:
return $content->item();