The term "Pound for pound" has nothing to do regarding a fighter actually competing at various weights during their career. PfP relates to ranking fighters abilities regardless of weight. So in other words looking at the skill sets of fighters ignoring weight divisions.
I don't remember any next-day report mentioning that Fitz-O'Brien II was for the light heavyweight title. If it was, none of the reporters seemed to care. The only mention of "light heavyweight" was in Billy Jordan declaring Fitzsimmons the middleweight and light heavyweight champion of the world.