A kilt is "culturally related" attire. No different than "African" shirts or other an Arab tunic. It is not legal in the US to discriminate based on culture, and if the OP has had positive performance reviews in the past, the manager would be very foolish to start problems now. If a kilt is not against dress code, then it's not against dress code, no matter what the manager says. If other employees can wear "cultural" based clothing, then all employees can as long as it falls within the bounds of the rules. End of story.

Many people's "African heritage" is 150 years old, but because of their skin tone no one argues with their clothing choice if they choose to wear those garments. You do NOT have to be from Scotland, nor have any relative there in recent memory, if you identify something of yourself in Scottish culture then you have the right to wear the appropriate clothing.

I'm soooo tired of the "some people have rights (but a white male doesn't)" stuff.