Hi Damien,
I think text in chapter 53.1.6 is complete.
However used clock terms in whole FlexCAN chapter could be little bit misguiding.
The protocol engine clock (PE clock, CANCLK), that is used for CAN bit timing specification, can be connected either to Oscillator clock or to Peripheral clock, selected by CTRL1[CLKSRC] bit.

The chapter 53.1.6 describe just the Oscillator Clock source, which is SOSCDIV2_CLK in this device.
The Peripheral clock is not specified this way and it is only visible from Table 27-9 and it is SYS_CLK.
BR, Petr