Legally, with respect to the doctrine of terra nullius, it is. The Brits didn't abandon the islands nearly that long - they simply didn't see the need for permanent settlment. The Spanish removal of the plaque is an acknowledgement of its legal significance. If the Brits never contested challenges to their sovereignty, you'd have a claim for implicit abandonment, but alas, the Royal Navy always showed up and asked politely.
The doctrine of terra nullius never had a time limit under either Roman law or admiralty law for a sovereign to contest an unauthorized occupation or claim against the sovereign. So the Argie claim only exists de jure if the Brits said "that's ok, we don't give a ****"
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