This is what I understand a "departementsjurist" to be. But, as you point out, if this is a promotional text for a firm of solicitors, then I doubt that this what is really meant. I got only a few hits with the term, one of which you can read (see refs. below). It sounds to me that what is meant here is lawyers who have expertise in certain areas of law (e.g. planning regulations). Or perhaps that they use the services of ministry lawyers - which I doubt would be considered ethical..? I would have checked with the client on this one.