No, it is merely a hyperbole. You are right in the legal sense, the maintainers don't owe him to put his name in the copyright header. There is no legal IP for the root cause analysis part of the work.

However arguably root cause analysis is a significant part of fixing a bug, often much more work than writing the lines that fix the bug. A "reported by" is not an acknowledgement of this kind of work.

IMO the author earned an opportunity to write the patch themselves with the guidance of the maintainers. What the author got robbed of is this opportunity.