I've still never quite understood the difference between an FDE and a contractor SWE the client hires through their parent consultancy, other than that with the contractor it would be clearer the hiring company owns all the work, data and infra produced.
Does sound like a nice perk. Having done the SWE consulting thing for a stint, you do start to dream of having lethal force as a backup for the more unpleasant clients.
Pessimistically I might think that the client demands data privacy and ownership on paper, but also doesn't want the liability of actually owning and maintaining or even touching their own vast stores directly. While the firm would love to learn from the data to better their systems... and more. So there's just a nice wink and a nod about what's not happening. But that is an uncharitable conspiratorial lens. I'm sure it's fine.
