That's surprising as it certainly feels like more of it is restricted when driving on it. If you drive the autobahn you will curse that any of it is unrestricted. There is enough changing between limits and no limits to create additional cognitive load. It seems like more than 30% is restricted as time traveled on the unrestricted bits is less per km. I never received more speeding fines in my life than when I drove from Berlin to Austria and back.
A lot of German highways do have speed limits. I have driven from Norway to the UK and back several times over the last three years via Germany, I think that probably only half the motorway driving in Germany was unrestricted. And even where it is technically unrestricted the traffic is so heavy during the rush hour that you can sometimes walk faster. I have Hamburg and its perennial roadworks in mind.