Ok, let's play incorrigible pedant. I'll go now, you can all take your turns in the comments.
English ain't algebra
English is not algebra¹, and treating it as it it were is a category error. This is the primrose path they're always going on about.
Case in point, @jejorda2 gives the correct answer, but for the wrong reasons. We don't answer grammatical questions with arguments from physics, just as we don't justify physical theories with arguments from grammar. Which is worse, you tell me.
Let's get on the same wavelength
The word countable (not enumerable) in the rule you cite refers to grammatical number, not cardinal number.
That is, just as to a chef a tomato is a vegetable but to a botanist a tomato is a fruit, so to a mathematician frequencies (the physical phenomena) are unenumerable, but to a syntactician frequency (the word) is countable.
The word and the phenomenon are not the same thing.
Incorriging future pedantry
Quick sniff test for future cases: if you can pluralize it², it's countable. If you can't, it might not be countable (i.e. mass), but there are edge cases (like plurale tantum).
¹ I mean "English isn't algebra" in a much broader sense than "it doesn't deal in the countability of infinities": I mean it in the sense of "English is a language: a messy, chaotic agglutination of artifacts produced by millions of people over centuries, in an ungoverned and ungovernable manner. Trying to apply algebraic transforms and expecting algebraic consistency is so misguided as to be "not even wrong".
The key takeaway from this message is that English is under no obligation to be logical. None at all. And it often uses this freedom. Sometimes, of course, it is perfectly logical, as in this case, where just because something has the potential to be infinite doesn't mean you can't count how many there actually are, as @Yosef Baskin points out in the comments. There are an infinite number of books in Borges' library: would you like to make the argument that book is uncountable†?
But to be clear, this particular instance of logical consistency isn't the basis of the justification of frequency (the word)'s countable status. The fact that people pluralize it is. English cannot be captured in closed and comprehensive set of rules. If you did that, it would just break those rules and make new ones. The best we can do with English is describe what we observe.
² Note: not if it is plural. That's a different test.
† I know, I know, the Library has measure ℵ0. Sigh.