If my math is correct the plate is over 1 and 1/2 inches so you have to preheat it. Ask your friend where the foot note is that says "If your WPS is qualified without preheat ignore Table 3.2."

Tell your friend to look at the UT acceptance/reject chart.The 5db flaw that fails on 3/4"(19 mm) plate regardless how long it is, passes on a 13/16(20 mm) plate even if it is present from one end of the weld to the other.

Read this:

"4.7.4 Preheat and Interpass Temperature. The minimum
preheat and interpass temperature should be established
on the basis of steel composition as shown in
Table 3.1. Alternatively, recognized methods of prediction
or guidelines such as those provided in Annex XI, or
other methods may be used. Preheat and interpass temperatures
lower than required per Table 3.2 or calculated
per Annex XI may be used provided they are approved
by the Engineer and qualified by WPS testing."

My interpretation would be that either the temperatures in Table 3.2 or alternate temperatures of Annex XI could be used for the thickness to be welded. The use of a lower preheat than either of those for a given thickness would not be prequalified and would need to be demonstrated as producing sound welds for that thickness. As the Engineer, a PQR on 1-1/2" or greater thickness of material is recognized as having the same cooling rate as an infinite thickness plate, therefore I would be looking for a PQR on 1-1/2" or greater material with the proposed reduced preheat. D1.1 is not very clear on this, so you're going to have to look for the "Engineer's" approval to do this. You would not have much success getting approval for this where FEMA requirements apply.

You would also need to decide if this is a battle worth fighting for every job you use this WPS for. While it may be technically acceptable within the wording of the code, it takes time away from other activities to explain the basis for each customer or inspector that questions it. That time may be better spent on other work.