The context of that statement is in actually measuring IOPS... What they are saying is that the number of I/Os processed per second is a measurement of it's own and is not dependent on the size of the IOs themselves. ie: 1000 IOPS @ 4KB = 1000 IOPS, 1000 IOPS @ 32KB still = 1000 IOPS. As IO size varies at a given IOPS, bandwidth will be affected.
Where IO size matters is as size climbs over 16KB and into the 64KB+ territory, the maximum number of IOPS sustainable by a given storage system will decrease. ie: IOPS and Bandwidth are inversely proportional.