I stopped reading when the author claimed to use WAAS in Australia in 2011
And the difference between GPS Altitude and Baro Altitude is a red herring anyway, because what we mostly care about here are the altitude
changes, not the absolute altitude value be it GPS or Baro.
Which is why the experiment RYFQB did (see
chart a few pages back) was neat because it showed that altitude changes computed based on the FR24 GPS vertical speed matched very closely with the recorded pressure altitude changes.
So even though the FR24 pressure altitudes are unreliable past a certain point, we can reconstruct corrected altitudes with some confidence by using the computed GPS data (which obviously is not affected by pressure data errors).