"Increasing prop pitch" to describe a lower blade angle is very rarely used by pilots.
Increased
prop pitch? Or, did the pilot intend to convey propeller control "increase" [RPM]? The pilot wrote that he "cut the throttle"? What did he mean? He cut the throttle
effect, which effect would
reduce power, so power was increased? Or, he cut back the throttle control, which has the effect of throttling the engine, and reducing power? The language is skewed in two places in the report, why focus on one language error, and not the other?
In reading the original report, if that were being reported to me by a competent pilot, who was probably still justifiably excited reporting such an encounter, I would interpret what the pilot meant to say to be that he moved the propeller pitch control to the "increase" position - which is a
lesser blade angle. Interpreting that the pilot rapidly reduced power, and extended some flaps, there is no plausible reason to then
coarsen propeller pitch. I bet that if you asked the pilot did he mean to say that he "increased the propeller
RPM?, he's say: "yeah... that's what I meant". If power is being reduced in maneuvering, or anticipation of landing, you want the prop to be in fine pitch. If not, if/when the power is increased, perhaps quickly, and to a high power setting, a constant speed propeller would govern to a lower RPM, and cause damaging overboost of the engine.
For the P-51, the propeller control is labelled: "RPM" "Increase" (forward), "decrease" (rearward). This is because that control controls the propeller governor, so the pilot selects RPM, not pitch - the governor controls propeller pitch in the governing power range. At low power, the governor no longer governs, so then the propeller control can move the blade angle to fine pitch. The P-51 Pilot Operating Instructions do state that on approach (so power reducing, flaps being extended) the propeller is to be set to 2700 RPM, which is the maximum continuous RPM, so increase RPM, decrease pitch.
For the information which you are selectively interpreting from the report, while overlooking some simply realities of piloting constant speed propeller planes, I think you're leading yourself off the track somewhat.