STM32 SYSCFG is a weird thing. It just seems some kind of catch-all name under which some random registers were put in absence of better placement.
I'm not surprised at all that some of these registers seem to work without enabling the SYSCFG clock. It's probably something like the SYSCFG-located EXTI configuration registers being clocked from EXTI and not SYSCFG; i.e., maybe the registers are, after all, in one place, with rest of EXTI registers (EXTI_IMR etc.). If this is the case, it's beyond me why they are documented as being part of SYSCFG, but it would be typical ST. This is just a guess though.
But don't count on it working reliably, or with every device.