I think the authors point was that even though the mainframe was underpowered from a raw performance point of view, it augmented that performance with accessory

I think the authors point was that even though the mainframe was underpowered from a raw performance point of view, it augmented that performance with accessory units and processors. By offloading as much of the non-core functions (I/O) as possible, the machines themselves would be able to process as much data as possible without waiting for input. So while software has gotten more complex/bloated and hardware ludicrously fast, we still are waiting for input, IO, network, etc. I think of it as the old school mainframe engineers knew every trick to eke out every ounce of performance. And they used all of the tricks. We’ve sadly lost a lot of those tricks because our hardware is so fast, we can ignore the performance losses.