A ninja does not self-identify as a ninja.
In our field, a lot of folks will call themselves "ninjas" or "10x devs" or "experts," but they're not always terribly correct in doing so.
The simple truth I have found is that if you are in fact an expert, or a "ninja," (leaving aside the really problematic ideas of a 10x developer) others will tell you. There's no need for self-identification.
Accordingly, if someone does self-identify as a ninja, they probably aren't. A ninja does not self-identify as a ninja.