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.
I grant you Sumer and possibly Egypt. Though Upper Egypt totals about half the arable land of the Delta, and I suspect the swamps of the Delta were harder to bring into cultivation than the intermittently flooded lands of the Valley.
However, civilization in China definitely began some hundreds of miles inland and didn’t spread towards the coast for many centuries.