This rumor is coming via two very old and long time rumor source. These kind of sources do write me anonymously only 2-3 times a year. They were correct 95% of the time with only one small miss in their long record. An example of their shared rumors can be read here (Sony A9, 100-400mm FE, 85mm FE, 100mm STF, 16-35mm GM, 135mm Batis)

Compared to the two sources of the wrong A7III and a7sIII launch time date these sources are of a much superior level of reliability. Still I will rate this rumor with SR4 (70% chance to be true) and not SR5 because I wanna be a bit more conservative than I have been before. SR5 rumors will be posted when I am nearly sure Sony did not spread wrong info intentionally.

Here is what they said about the new A7sIII:

Source 1 (with a track of sharing 5 correct rumors out of 6)

4K HDR 60P

The sensor is Exmor RS

The number of pixels is unknown.

Sony is nervous about information leakage, it is difficult to obtain detailed specifications.

But according to sources, the A7S III is amazing, it is really amazing.

Source 2 (with a track of sharing 4 correct rumors out of 4)

The sensor of a7sIII would be new stacked CMOS sensor with integral memory

which means a7sIII would have 4KHDR/60P.

The sensor is technically superior to a9/fs sensor.

I Thank these long time source for the diligent work. I hope he can get some detailed sensor info soon too :)