I had this on a SAC too. It’s misleading at best.
Entering a built-up area, you’ll pass a large 30mph sign which defines the limit. You will then have either regular repeater signs or there will be street lighting present. The street lighting acts as a reminder and negates the need for extra signage, it does not in and of itself define the limit, that was done by the last sign you passed.
With an NSL area, the opposite is true. You will have repeater signs if there is street lighting present. Again, the absence of lighting would mean that repeater signs were unnecessary.
I was taught this on my test. It was framed as a handy tip “if you’re ever driving along and aren’t sure of the limit, and there’s no repeaters, look for street lights.”
The limit for the road you’re on is defined by the last fill-sized limit sign that you passed. That’s it. And unless your car arrived here by teleporter you will always have passed a sign.