I think you’re a little bit confused.
The Egyptian civilization in a flood plain up to 800 air miles from the coast, at least via the Nile. The Two Lands in Egypt were upper and lower Egypt. Upper Egypt was the river’s narrow floodplain, Lower Egypt was the delta.
Mesopotamian civilization developed in areas up to 600+ air miles from the sea.
The first Chinese civilization was centered over 400 miles from the coast.
All of those are still relatively coastal, in that the most fertile parts of the land were near the coast, and the area of arable land dwindled the further inland you went. That’s dictated by pretty basic principles of geography. The further down the river, the wider the flood plains tend to be, and the more nutrients will be deposited, as the rivers slow down as they get wider, and a slower current can carry less sediment, so more gets deposited on the banks. Saying there were still some narrow arable areas hundreds of miles inland does nothing to detract from the fact that most of the arable land from those civilizations was actually near the coasts.
Seriously, just go look at the satellite maps of any of those areas. The arable lands are still visible to this day following that pattern, and it holds true for most of the river systems on the planet.