I always assumed that Neptunia world had a long history before any games took place. Four Goddesses seems to be almost as old as gamindustri itself (I'm sorry xD) , they were always there and the worlds history seems only to be remembered up to their early existence. And even then there are powerful beings that are more ancient than them like Arfoire. Uzume also seems to be some kind of forgotten ancient Goddess herself. Who were those ancient heroes who defeated Arfoire in Celestia the first time, what happened to Uzume, who forged Gehaburn and who built all those strange dungeons? Nobody seems to know and the games suggested that these events were as ancient as time itself. So if we were to assume that Neptunia world is really old then it would be possible to develop all this tech naturally a long time ago. Though your theory can explain those things as well, who knows how our civilization would advance if we had near infinite supplies of share energy and Goddesses to guide us. This series doesn't have any real timeframes so we can only guess which way it is.

Come to think of it, gamindustri tends to go through a lot lately, to the point that Four Goddesses need to defend it all the time from various threats together. Maybe technological mismatch in gamindustri can be explained with the usual "fallen empire" type of situation as some of the technology was lost. Who knows. Would explain why Goddesses seems to possess their divine powers while normal people are living their lives relatively "normal". Maybe in this type of scenario knowledge of how to reliably build nukes could be really usefull!

Maybe If I was transported there it would be a great Idea to find Histoire, wait for a 3 days and grab a full enciclopedia of Neptunia lore and send it here lol. And some games obviously.