[k1t]

I assume it's because you could read it as: the union leaders view this big union victory as an existential threat to labor standards.

(When instead they view Amazon as the existential threat)

[dfxm12]

I disagree that any reasonable person could read it that way. It's totally impossible to come to that conclusion with the context of the article. Even out of context, it makes no sense that union leaders would feel a union victory is an existential threat to labor standards.