I agree, the description is not very helpful.
> ...the system clock is set to 172.9 MHz ...
This is just a variable, and by no means proof of the actual core clock frequency. Setting up the external clock source might have failed, and thus the default clock is still active.
When I came across such instances, I configured the SysTick timer to a nice round ratio, and toggled a GPIO pin in the hander routine. The measured frequency must correspond to the configured core clock.
The SysTick timer/code is CMSIS functionality.
/* one interrupt every millisecond */
if (SysTick_Config(SystemCoreClock / 1000) != 0)
{
// whatever error handling
}