In the short term they might not care, and for an old civilization the short term might be a few hundred years. If they are anything like current day humanity, we would end up interacting with a tiny portion of their population like some middle manager at an interstellar mega corp or xenology students on a grant from their nearest university.

We won’t be important, and certainly won’t be making any appreciable difference good or bad to their civilazation as whole.

Worst case scenario that I think is plausible is that they are so decentralized that one of their warlords would come over and set up an empire on top of us for a few hundred years before the central authority realizes anything.