Exactly. While patent and copyright are two very different animals, the real issue is that your employment agreement (or your independent contractor agreement) will likely have an "intellectual property" clause whereby you assign your rights to the work, or at least give a license that's almost equivalent to an assignment, to the employer, if the work is made within the scope of your employment or using knowhow you gained on the job or employer assets. This stuff is pretty standard in arrangements where intellectual property might be created.
The real issue is what Kirby's agreements said -- if anything -- on the subject. You only really have a case where there's bad drafting, nothing written, or some other kind of loophole. But if the agreements were drafted properly, Kirby's estate doesn't have much of an argument.