The word "expert" has a lot of different meanings. In court, an "expert" is a person with specialized training and/or experience beyond that of a lay person which enables and allows them to give opinion testimony rather than merely state what they saw or heard directly. An expert may be able to take a skid mark, and state that, to leave a 30 foot skid mark on that pavement, the car had to be traveling at about 25 mph. Or that an officer's particular use of force was reasonable and appropriate to the resistance encountered and in keeping with the agency's policies and laws.
Another way to define an expert is as someone who knows nearly all there is about a subject. A third, similar definition would be a person with a high level of skill in performing tasks or actions.
The simple reason that I say that it's hard to impossible to be an "expert" in self defense is what I laid out earlier: few people encounter enough situations where they use self defense skills to be able to reliably assess something that worked once, something that was pure luck, from something that is reliable. That's not to say you can't learn enough to have a good guess, and to develop sound assumptions, or to qualify as a court expert. I just dislike the idea of labeling myself as an "expert" colloquially in a subject that is so subjective in experience.