I think what they're getting at is if you have the proper exposure protection (what works for you and with the proper fit) you shouldn't be cold NOR too hot. It may well be that a slightly thinner suit than a 7mm would be appropriate. And as mentioned before, what works for one person may not necessarily work for another. Of course, if you go too light you'll get cold.desert camel:Fine. you are the first and sb_diver is the second I heard of feeling too hot in cali water. Anyway, you can always unzip to cool yourself down. But what can you do when you feel cold?
For me, I have no problems whatsoever in 57F water in a proper 7mm. However at 70F, I would definitely be overheating. I don't know if SoCal gets that high in the summer, since I've not dove there in the summer.