This was proposed by Zhao Yi.
Photon antigravity was his research achievement, and he had a deep understanding of its complexities. Since the internal design was extremely complicated and there was no theoretical foundation, even if the antigravity device were given to other countries for research, at most they could mimic and create something roughly the same, yet they would not be able to advance any further in their research.
Therefore, owning the foundational theory would enable dominance in the research and development aspect.
This dominance was extremely advantageous for the nation and its future. However, the antigravity technology itself couldn't directly bring about benefits and would still require a significant investment in research and development.
An aerial fortress would require substantial funding to build, and even once constructed, it would at best significantly enhance near-coastal defense capabilities.