It's all about as trustworthy as the mythical high salaries for retired mainframe developers. I've never seen a single job posting nor any other proof that this is indeed true, in fact I've saw evidence to the contrary.
People in these roles don't want to draw attention on their sweet comp packages which is why you won't see them crow about who/where they work.
I have a friend who went from being the form to the latter and immediately doubled his salary, despite the consulting firm making it clear that he was at the bottom of the totem pole, despite his being 5 years or so into mainframe work.
I suppose the point is, these jobs exist but it's quite hard to replicate as it's not common with large firms that are regularly hiring.