Obviously. But if there is 400 grand on the line, you'd think someone would actually check(when the claim is made). The receiver would say "you have a signature from person X. Person X doesn't actually work here". Fedex then says "ok, prove it" - and then the receiver does, in whatever way is legally acceptable.
Edit: in fact, let me add a bit more - if the shipment was delivered to the right address just signed by someone who didn't actually work there then sure, I think FedEx would be in the clear. But they delivered the parcel to the wrong place - the fact that it was signed for by someone is almost irrelevant, it's the same as having no signature at all.