This is why 366.58(B) should have never been added to the Code. It gives the impression that CMP 8 doesn't know what an auxiliary gutter is. The addition of this text during the 2005 Code cycle further blurs the lines between gutters and wireways. By its very definition an auxiliary gutter is used to supplement the wiring space of other equipment. If used as a pullbox it would be a wireway and not a gutter. In the photograph the gutter should be viewed as supplementing the wiring space at the transformer, therefore 366.58(A) applies. According to Table 312.6(A) the gutter provides sufficient space for this purpose.
I disagree with requiring the derating of conductors bundled together in the gutter space of a panelboard, that is unless there are more than 30 current carrying conductors bundled together. If you interpret the code to require the derating of these conductors, wouldn't 300.3 prevent their installation in the panelboard anyway?