Yes, it is an unusual rule – that is why I mentioned it. Not many potential buyers know about that rule.
Guests can park in your driveway or garage overnight – they just can’t park on the street overnight.
Also, there are a few (less than 10 as I recall) guest spots just outside the Stone Canyon gate. That may or may not be inconvenient, depending upon how far your house is from the gate.
On the topic of San Marcos, the paper today states that the city has $50M in reserves and should do just fine this year with no budget or staff cuts planned. They won’t be able to hold up forever, of course, but are currently doing much better than many other cities in San Diego county where city staff layoffs could number in the hundreds. I’ve had my issues with the city of SM, but overall I think they are doing an admirable job of managing the city. Something to consider when house hunting.