A lot of microaggressions are assumed to be about one thing but are actually something else. Maybe the white woman clutched her purse because he was a man. Maybe she was just moving it closer in a tight space. Maybe he reminded her of someone specific. Sure, it is probably race and if the same person does it multiple times it could be confirmed. And yes, it is totally reasonable for a black man to see a bunch of white women clutching purses and assume there is a pattern even if not every single one was due to racism.

Microaggressions are one of those things that are real while also frequently misread because it is impossible to infer intent from a single vague action. Better to assume the minority/oppressed group is right, and if accused of something in error just let it go.