Yes, it is an adjective functioning as an adverb ("magnánimos", masculine.plural, is concordant with "hombres"). It happens also in English when you say: I feel "good" and not "well". It is not a strange Argentine dialect, it is literary Spanish. :) In that sentence to use magnánimamente would be also possible. Another example: "La familia vivia feliz en el campo", instead of ´" La familia vivía felizmente en el campo".

John, here you have a link with an explanation of this phaenomenon: http://zonaele.com/adjetivos-en-funcion-adverbial/