Rva, the citation you quote is a state-level quote. Therefore it would only apply to state-level land (ie.: state parks, etc....). Not cities and counties, which generally have nothing at all specific to metal detecting. And if it is silent on the issue, then SO BE IT.
I'll bet if you were to look for yourself in the county rules where you live, to see if there's some sort of prohibition on metal detecting (like the lady you talked to says), you would find that there is no such thing. Does the county have a website? Go to it and see if there is a codes, laws, or regulations section. Do a key-word search on variations of "metal detectors" or "detecting", etc... I'll bet it's silent on the issue. If there's no website, the laws, codes, and ordinances should be available on the front desk of the county offices (since it's public domain info). Search through the park rules section of the books, and I bet there's no such rule.
Your situation of "calling the park office" and getting a "no" is nothing unusual. Oh sure, they might even add: "it's a crime". Because you see, the mere phrasing of the question in the form of permission (ie.: "can I do such & such?") indicates to the person that there is something inherently wrong or evil about you or your hobby, that you had to ask permission to begin with! With that implication in mind (not lost on the person you are asking), a "no" is the easy answer. Afterall, if there were nothing wrong with your hobby (like flying a frisbee) you wouldn't have asked, to begin with, right? Therefore, in their mind, they're going to imagine geeks with shovels, or think they must apply state-level laws, to apply to your "pressing question". Odds are, no one would ever have cared or noticed if you'd simply went, unless you were a nuisance in some other way, sticking out like a sore thumb begging for scrutiny, etc....
I have seen this pyschology played out again and again, in places where people routinely detected ........ and no one ever cared. Until someone asked.