It is, of course, a big what if the war went well for Germany.
Germany was building an aircraft carrier during WWII (the Graf Zeppelin) and had plans to build others.
Germany did have a long range bomber (Heinkel He 177 ) and long range airplanes (Condor). They had plans for long range jet bombers and long range ICBMs.
Germany was also working on the atomic bomb and probably would have developed it in 1946.
Throw in captured and salvaged capital ships from France, (assuming) England and (assuming) Russia, and yeah, it could have happened...
Why would they have needed long range bombers, fighters, an aircraft carrier, or a navy? The author eliminated the standing federal military, so we don't have a navy, and nothing with which to prevent a seaborne invasion. No air force either. Just a bunch of "rednecks with rifles."
But even if we did have that stuff, the Germans wouldn't need it anyway. They'd just land in eastern Canada without declaring war on us. And of course, we've got no military to help the Canucks, so they'd go down pretty easy. Ship over troops in transports, set up a nice base of operations in Canada, and then roll into the U.S. when ready. I'm sure our citizen militia would have done great against the Wehrmacht.
If the Germans were really clever, they could send a bunch of weapons to the Mexicans, and promise them Texas, Arizona, New Mexico, and California if they help us out. Of course, with no standing military, we would lack the ability to intervene in Mexico too.
It's numbskulls like that author who give libertarianism a bad name.
Let's look at actualities instead of fantasies about how Germany could be a threat to the US. Germany was engaged against England and Russia. They were under strength when they went into the Soviet Union in the first place. Because of this weakness they could not finish the job in 41. By 42 they did not have the strength to continue the assault along the entire front so they held in the north and center while pushing only in the south. Germany was way over extended and in a death struggle in Russia. Invading the US was not a consideration or even possible even if they had the inclination.
Developing the technology for long range planes is not the same as having fleets of them. As far as cross Atlantic invasions go we were in a much stronger position in 44 but still we could not have pulled it off if we didn't have the land mass called England as a huge base and supply depot. Germany would have had no such advantage.
As for atomic weapons and rockets and supposing Germany had conquered Europe one still has to be realistic. What is the difference between Nazi Germany and the Soviet Union? We deterred Russia so why not Germany?