I was concerned about this also. According to the AIG Discord, the installation program renames a file in the official file structure to disable default planes.
It renames trafficAircraft.bgl to trafficAircraft.bgl.aigoff
in the …\Official\OneStore\fs-base-ai-traffic\scenery\world\traffic folder
To re-enable the default traffic, you remove .aigoff from the bgl file.
I have not tested this myself.