Uh, not exactly. California is, uh, different. In many ways, including speed limits. Some would call California "special" in the SNL sense.
California law calls posted speed limits "pima facia". But Calif's "basic speed" says you can't drive faster than is "reasonable and prudent". I call it fitting' and proper. Under case law precedents, posted speed limits can not be less that the 85th percentile speed as measured with an traffic study, unless accident history suggests this is unsafe. Exceptions exist for school zones and such. (So when we see those two rubber tubes stretched across a road, we drive extra fast so as to raise the 85th percentile.)
So, when California Alan says that particular "speed limit" is not enforceable, what he actually means is that a ticket for violating the posted limit may be successfully contested. So police instead cite for reckless driving, which includes driving 15 MPH over statutory maximum speeds, such as 65 on freeways or 55 on undivided two lane highways, unless posted higher in either situation.