That can't be right, unless they meant the PPC604 as opposed to the PPC604e. There were zillions of 200MHz PPC604e machines shipped with a 50MHz bus speed. So clearly 4X clock multiplier was available. IIRC, 3.5X was available as well.
I looked at the link to the PIOS. I think that the 604ev at 300 MHz statement and the dual processor statement are independent of each other. In other words, the PIOS comes with a single 604ev (interesting in itself; I thought only Apple shipped those) but the base machine has the ability to use dual processors. This is emphasized on the Umax S900 clones (clone of a clone) because the motherboard has two CPU slots. They're not actually any more capable of dual processors than any other X500 (PowerSurge) based machine. It's just more obvious.