There is a very simple reason that we can say with relative confidence that there were no earlier civilizations that vanished and that reason is domestication.

There is just no evidence of plant or animal domestication before a certain date range and, while that date range does keep getting pushed back, it doesn’t get pushed back in a way that suggests any sort of civilization even as advanced as Sumer existed before Sumer. It gets pushed back in the “they were planting and harvesting this crop but didn’t know how to make it very nutritious yet” sense.

We can see based both on morphology and genetics that there’s no sign of any sort of civilization that domesticated plants and animals which then went feral after the civilization collapsed and, even with massive sea level rise, there should be some evidence. Sea levels didn’t rise all of the sudden. There would have been people who had time to escape with their animals and seeds. Also, plants just have a habit of escaping on their own.

You need farming in order for a civilization to advance. You can’t feed a large population via hunting and gathering.

While I don’t doubt that there will be some genuinely ancient stuff now underwater, it seems unlikely that it would shift the global picture of the emergence of settle agricultural societies that much. Most “cradles of civilisation” are inland river valleys - Tigris, Euphrates, Nile, Yellow River, Yangtze, Indus, Tehuacan - with the exception being sites in Peru. Being by the coast only becomes useful once you get good at building ships, after all

I’m not in any way actually qualified on this though, so if there’s some actual research saying otherwise I’d be delighted to read it. There really was a lot of sea level rise in the ~10,000 years before we know that agriculture got going, so it would make a lot of sense that at least some stuff got flooded