[narfforc]

I think that Elrond would have learned of this Battle from other sources, the comings and goings of those from Aman and as we read of things other than our experience, we imagine how great events unfold. Elrond's memory of The Battle of the Last Alliance, would be tinged with this story he had read. Imagine an old relative or friend tells you of a past event, and then you experience something similar, you may think of the past event. The other explaination could be that the army's march through Beleriand was witnessed by many that never went to the battle, (and Beleriand was ablaze with the glory of their arms)