[scoot]

What seems to be the problem?

[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.

[brimble]

There are multiple things in the first part of the sentence that "which" could denote. "Amazon" is what's intended. You could fix this by breaking up the sentence, or by specifying what you mean after "which" ("which company").