I think you are missing the point. What the Germans wanted to do, and what they considered your brother to be, is completely separate, absent some provision of the "status of forces" agreement, from what the US government considers him to be.
And just what did the Germans consider him to be? Certainly not a German citizen, since Germany has a large population of native born non-citizens, most of them of Turkish ancestry. I met one of them a year or two ago. A most fetching young wench, and apparently a very sharp engineer as well.
That waiver process was likely the US government intervening on behalf of one of it's citizens with the German government.