The biggest difference is that in Germany, the speeds are more sustained. In the US, cars generally drive in whatever lane they feel like, so your speed is constantly changing. So even a difference of 180 vs 140 will be significant. (and it could easily be 200 instead of 120).
While Germans are a bit more conscious of driving in the left lane when not passing, what you are saying neither makes sense, nor is it applicable, especially in any amount of traffic.
It also totally discounts the vast amount of road construction and maintenance that Germans continually do on their roadways (infrastructure maintenance is a big deal). The construction alone, especially in Hessen/Rhineland Pfalz vastly impacts travel times in ways going 40kmph faster could never attempt to approach.
If I choose to drive 120/130 on the autobahn I have a much higher chance of doing so continually than if I choose to drive 75 Mph on an Interstate highway in the US.
I'll give you my anecdotal evidence from last year's trip to Germany. I took a ~550 km trip, and while I don't have actual GPS data from my trip on hand, I do remember that I was able to drive roughly one third of the way in sections with no speed limit and no traffic/construction.
For 2/3 of the way my average speed (including traffic and construction) was 122, so that took 3 hours. For the rest (1/3) of the way I was able to travel with unlimited speed. Traveling 183 km at 183 km/h (average) vs. 122 km/h saved 30 minutes of my trip.
Obviously construction (anywhere) affects travel time. But in the US the highways have more exits, cops running radar, and there's ALWAYS someone going slowly in the left lane. Unless you're in the western desert states, it's pretty hard to maintain a steady speed. By contrast, everyone in Germany knows how to drive, and everyone follows the rules about keeping right and passing left.