The etymology of the Portugese "marmelo" is clear, from Latin "melimelum", itself from the Greek "μελίμηλον", meaning "sweet apple". The product was famous, so the word was borrowed to sell orange paste, later orange jam.

I already had a glimpse of the story thanks to a chance reading of the etymology of the French "marmelade", but I enjoyed reading the whole story.

https://www.littre.org/definition/marmelade (~1875) / https://www.cnrtl.fr/etymologie/marmelade (~1980)