external power treatment in the powered board
Let's say the load of the AUDIO POWER is around 4 ohms. And the requirement is to have the power rail changes no more than 5% with 100% changing 4 ohm load, therefore the power supply impedance needs to be less then 4 ohm x 5% = 0.2 ohm.

Further assume that the power supply is an unknown cannot be relied on and the 0.2 ohm impedance is to be satisfied with on-board capacitors only at 40Hz (lowest audio frequency desired).

C = 1 / (2 * pi * freq * 0.2) = 0.02 F = 20000 uF (I could be off by a factor of 2)

So at first glance, the capacitors seem way overkill, but there are conditions where they can be justified. It really depends on the load, the power supply and the requirements.