There’s no reason there has to be peace for a thousand generations. Ben just says the Jedi were the guardians of peace in that time.
Palpatine in AOTC says the Republic has stood for a thousand years. Potentially a discrepancy between that and Ben’s line, but you could explain it. Either way neither of them says there was no war at all for that long.
Ah, nice. So going off that we could say:
The Jedi have stood for a thousand generations, acting as a separate religion, and when the Republic was formed they became their guardians of peace and justice. It makes sense for Ben to group the two together when explaining to a young boy with a limited attention span.
The Republic was founded one thousand years ago, and could have had all the major conflicts Disney wants to fill their story. Personally I’m very excited to explore this period.