My speculation is that Ekko is just using his own terminology. Whether Zaunites themselves or people on the outside use the established term "Zuanite" doesn't matter to him.
To him, if you come from Zaun you are a part of it as much as it's a part of you. You're Zaun.
EDIT: Further speculation is that it's also an endearing slang word/title. Since the interactions with Vi/Viktor.
@ Vi: It's like he's recognizing her for it and not just categorizing her as just another "Pilt-y".
@ Viktor: He's gotta prove it as if it's like a title to be proud of.