One major difference I can think of, between two conditions 1: filter in front of camera lens and 2: filter in front of an enlarger lens, is direction of light that'll be hitting the filter.
In case of camera lens and the filter, the light will be arriving at the lens from all directions including directly at the lens axis and at an angle, including nearly at 90 degrees if a hood isn't used. On the other hand, enlarger filter only sees light coming at it from one direction at lens axis which crosses filter at 90 degrees.
I am no optical expert. I know from experience, scratches on Ilford filter makes no difference. But, I also know, if I want to see the scratches on surface, I won't look at it straight on. I see them at an angle. Suddenly, scratches reflect light (thus I can see!) and become obvious. Not a condition enlargement filter will see but lens filter will.
I think we are comparing dissimilar systems.
Has anyone compared Lee or Cokin system verses regular filters? That'll be more similar. If you badly scratch those.... would it show?