The LM334 provides about 2.3 mA (as shown in the schematic) to the LM350's Vreg pin. Flooded lead acid batteries prefer lower charging voltages at higher temperatures to reduce gassing.
The temperature sensitivity of the LM334 (in this circuit, about 7 uA/ degK) is apparently used to compensate for this. One would hope that the LM334 isn't heated by the charger circuits and is otherwise in a similar ambient as the battery.
The website owner specifies a calibration temperature of 25 degC, which must be obeyed to make this work properly.