It's a completely acceptable explanation. Much more so than "it didn't exist at all" which requires enemies with thousands of nukes pointed at each other to conspire for 50+ years.

The issues are pretty straightforward, when the SaturnV and lunar lander were being built, almost all of the design was done by hand, all of the parts were made by hand and the engineers made all sorts of little undocumented adjustments to the designs in the process.

On top of that, the flight computers of the era were extremely primitive, large and heavy, and the design was done with this in mind.

Finally, NASA's safety standards were much more lax at the time. Saturn V would be considered way too dangerous to fly crew on nowadays.

Modern engineering methods are just too different to just recreate a Saturn V without effectively redesigning it from scratch, at which point it might as well be a much more capable vehicle like Starship.