>all we know about ancient times is what ancient authors have written
Contemporary approaches to ancient history do not rely exclusively on literary sources, but build a much fuller picture from epigraphy (inscriptions and graffiti can tell us a lot), archaeology, and numismatics.
You're right, not all knowledge comes from ancient authors. But I don't agree that there is enough information in epigrams and so on, to figure anything out with the context provided by long-form prose by authors who were sometimes even contemporary with the events they describe. I guess that's opinion though.
Contemporary approaches to ancient history do not rely exclusively on literary sources, but build a much fuller picture from epigraphy (inscriptions and graffiti can tell us a lot), archaeology, and numismatics.