That author is wrong.
From the current A319 FCOM:
When the temperate is lower than ISA:
‐ The true altitude of the aircraft is lower than the altitude that the ADIRS computes.
‐ The FPA that the aircraft actually flies, is less steep than the FPA that the ADIRS computes.
If appropriate, the flight crew should therefore apply corrections on the altitudes and on the FPA (in vertical selected FPA mode), and they should be vigilant on the parameters that are displayed.
....
FPA Correction
When the temperature is lower than ISA, the FPA that the aircraft actually flies is less steep than the FPA that the ADIRS (ISA referenced) computes. In vertical selected mode FPA, to correct the FPA for this ISA deviation effect, the flight crew should select on the FCU a FPA slightly different from the FPA that the aircraft needs to fly.
In any case, the check "altitude (corrected in temperature) versus distance" remains the reference.
I actually shot an RNAV approach in FPA at sea level at -27°C just 5 days ago. If I'd used the charted FPA (-2.54°) I'd have missed for sure.
I don't use the following in anger because of the variables but I got bored one day many moons ago and worked it out theoretically (cut and paste from my old notes):
•
H = FAFcorr – MINIMAcorr (in feet)
•
D=(Distance FAF to MINIMA )nm * 6072 (turns miles into feet)
•
On your iPhone calculator – turn sideways for scientific….
a)
Find H/D
b)
Press tanh gives angle in radians
c)
*180/pi
This is the Actual FPA that needs to be dialled into the FCU