Update:
I did an additional experiment.
I configured 2 products with a unidirectional link in a category and exported that category. When I imported back the exported file, the link was "Enabled" in both products, effectively making the link work as bidirectional, though "Bi-Directional" flag was disabled.
Then I edited the imported file in the following way. I just reversed the order of the two <P> tags in it. After I re-imported the edited file, the link in both products were "Disabled" ("Enabled" flag was not set in both products)
I did an additional experiment.
I configured 2 products with a unidirectional link in a category and exported that category. When I imported back the exported file, the link was "Enabled" in both products, effectively making the link work as bidirectional, though "Bi-Directional" flag was disabled.
Then I edited the imported file in the following way. I just reversed the order of the two <P> tags in it. After I re-imported the edited file, the link in both products were "Disabled" ("Enabled" flag was not set in both products)