Earlier quoted context omitted.
Well, the French settled forst and handed over the islands to the Spanish in 1776 based on the Spanish/Portuguese colonial agreement from the 15th century. For most of the time since the 19th the islands were under British control. That would be like basing territorial claims everywhere based on borders from 200 years ago. Or worse, picking the borders from a period that suits best. And we are talking about the Spani…
The issue is were they defending the claim. To do that you needed to inhabit it according to the laws at the time
Aside from specific agreements between states, there wasn't really international law at the time.