nobody would really know cause they both were full of tricks....
itachi was constantly doin new stuff out of no where...
and nobody knew wat jiriya could really do untill his pein fight....
so imagine wat else hes hidin around... so basically in shorts...
i think these 2 could have been natural foes to a certain degree...
jiriya didnt even know wat amaterasu was at one point... jus called it black flames..