More Words, Deeper Hole

As it's self-published, perhaps he can add a bit of backstory in version 1.1 to explain away the inconsistencies in the setting. Perhaps the low level of civilian technology is the result of a generations-long struggle to repair the Earth, which diverted all production and development to government use. Wormholes and their associated colonies as a desperate Manhattan Project, and were not intended as commercially-viable investments.

At the end of 1945, humanity had had ballistic missiles and rockets that could reach space (if not orbit), nuclear weapons, jet fighters, and huge transport aircraft that could fly across oceans. However, the lives of everyday people in Europe were still at levels seen in the late 19th century.

Textiles and manufactured goods were scarce and rationed until the 1950s, exactly the opposite of what we would have predicted during the boom of the Industrial Revolution. Food, even bread, was scarce and rationed, exactly the opposite of what we would have predicted at the dawn of agricultural mechanization. Houses were not comfortable and modern, but were cold, drafty, and suffering from deferred maintenance (and by some accounts, as much as 80% of German urban dwellings had some degree of damage). Domestic and street lighting was still largely provided by gas lamps, domestic appliances were rare, and only a small fraction of the population had ever flown in a propeller aircraft.

With a bit of handwaving, we could explain how we could have a future with a low overall technology level, but still have fun toys to move the plot along.