Totally agree. Leto II too many years later knew that the Ixians we're working on banned tech and the computers needed to calculate safe routes thru foldspace. Very interesting points about the smugglers. I bet they, just like the fremen, paid off the spacing guild to look the other way.
Unfortunately, Herbert was absolutely incoherent about space travels. Initially he gave the impression that Guild controlled the interstellar flights, but inter-system crafts (frigates, monitors, ect.) are under Great Houses control. Logical, isn't it? But then he completely confused everything, by revealing that without Guild help, Atreides couldn't even put their own frigate on orbit!
My speculation is, that Herbert initially thought in "Guild control interstellar, Houses control intrasystem" scheme. But then he suddenly found out (read in some magazine, most likely) about spy satellites. The existence of spy satellites, of course, completely destroyed his concept about no one knowing about Fremens true numbers. So Herbert was forced to quickly invent some explanation and squeeze it into already-existing narrative.
It's also quite possibly that Herbert understood little about spaceflight, and though that the orbit is something completely different from point-to-point spaceflight; so he may think that
"flying spaceship from planet to Guild highliner is just as simple as driving the car, but orbit... oooooh, it must be pretty complex thingamabob with all those frightening math formulas! Must be impossible without computers or Guild navigators".
Later he managed to mess situation even more by the addition of smugglers, who somehow are able to fly without Guild navigators.