“an existential threat” ≠ “a threat to all existence”
Simple semantics often slip up speakers. The phrase “existential threat” is used too often to imply that something described as such portents the apocalypse, in the way “communist threat” was used throughout the Cold War to as a casual substitute for nuclear annihilation. But “existential” is not a synonym for “apocalyptic” any more than “communist” was. Rhetoric and doubletriplepolitispeak are funny that way, though.
Semantic checkpoint: “Existential threat” implies the threatening thing threatens because it’s existential. To be afraid of something practicing Existentialism, then, for real, connotes a fear of what Sartre termed the “doctrine of self-making” where human reality is an individual, lived experience. Not very scary at all, really, unless making choices for yourself terrifies you.