Perhaps intimidated by that colossal spacecraft, the people who had still been cheering excitedly over the successful ignition of controllable nuclear fusion finally remembered the terror of being dominated by "Fortress X" and snapped out of their initial excitement.
There was no other reason.
The gap was simply too vast!
Even Earth's top physicists could not offer a reasonable explanation for the spacecraft's power source or propulsion method. At most, they could infer from scattered clues in the "game" that it probably used some kind of anti-gravity technology.
As for how exactly it achieved that, no one could provide a reasonable explanation.
And that meant human civilization's science and technology lagged behind theirs by far more than a little, starting from the most fundamental underlying principles...
The academic world argued from the four fundamental forces all the way to the conservation of energy, then from Minkowski spacetime theory to the ten-dimensional strings of superstring theory and the extra strings of M-theory.
While no physicists committed suicide because "physics no longer existed," the debates over new physics did make quite a few people lose what little hair they had left.
Some physicists proposed that perhaps a special kind of particle existed in dimensions invisible to humanity.
This particle obeyed not only the laws of physics in three-dimensional space, but also laws in other dimensions unknown to humanity. Thus, humans could use it to indirectly interfere with gravitational fields in higher dimensions.
Others proposed that the gravitational field itself might be a higher-dimensional particle.
To the point that humans, and indeed everything humans could see, were merely projections of that particle onto a three-dimensional plane.
Though these conjectures were incomplete and contained many baseless assumptions that humanity's current technology could neither prove nor disprove, they still counted as progress in a sense.