Yes. You are correct. The clock routed to the BLESS is different from the clock that runs the CPU which is SYSCLK.
To explain better how clock system works:
IMO and ECO are the main clock sources inside the PSoC. From IMO(/ECO based on the user setting) all the high frequency clocks are derived. For example HFCLK is derived from IMO/ECO. For every peripheral clock to be generated, there will be clock divider registers sitting in between the HFCLK and the peripheral clocks(SYSCLK also included in peripheral clocks). In the PSoC Creator, based on the clock frequency you set for the peripherals the dividers will be set automatically by the configurator. Once these dividers are set they will not change during the entire operation.
The peripheral clock accuracy directly depends on the source clock accuracy. For BLE applications the external clock is compulsory.