Somehow I don't think they would've written the headline "american astronomer discovers..." If the person had been american. To me it kind of seems like "oh look Spanish people trying to do science, so cute".
I think if an American discovered it, and you were reading a Spanish news site (originating from Spain, targeting Spanish citizens), it would not surprise me if the title were "American astronomer discovers ..."
Spaniard here. I think you are the one with prejudices, not the OP.
Thinking that of Spaniards when we were behind the first steps of CRISPR, having an astronomical observatory in Tenerife, making crazy revolutionary surgery on transplants in Valencia (and so on), that's a non-issue for actual scientific environments where they are more than aware on our skills.
HN it's mainly a US site, OFC it adds information to the news by pointing that an Spaniard did it. By just putting the surname of the author of the discovery, he might seem to be from any point at the US Southern to the Patagonia in Argentina or the Iberian peninsula.