Oh dear ... can you cite the report, please ? It probably warrants a looksee and comment to answer your question in more detail.
produced a ROC of 302 ft/min with 5° of bank into the live engine and 287 ft/min wings level
First up, 5 degrees relates to Vmc i.e., handling rather than performance.
At or around the optimum climb speed, a bank angle of 2-3 degrees (as a general rule) will produce the best result .. I don't know explicitly what the situation is for the KingAir but it will probably be much the same.
Climb performance measured against speed will look a bit like an upturned cup graph. Hence the importance of getting to and maintaining blue line.
Against bank, the graph is a bit more complicated as the direction of bank has a significant effect on slip. However, typically, for the 2-3 degree optimum situation, climb wings level and at 5 degrees should be somewhat similar and slightly less than optimum.
For the figures quoted, they are meaningless unless we know the rest of the test card point story .. speed, technique, etc. At this stage, all they suggest is the truism that climb performance varies with bank angle.
Does this mean that banking is only important to optimising ROC on FAR 23 types?
FAR 23/25, model aircraft, whatever .. all same same. Back in the lower speed end of the envelope, bank critically affects OEI climb performance. Ergo, you can find a bank angle to optimise climb. At higher speeds, the effects are much smaller and, in general, can be ignored. All to do with sideslip angles, both for the Vmc and climb performance case.