I do think "blog post learning" can work, but agree with you that this ain't it. The "explanation" on the blog post would really only work for someone who more-or-less understood Fourier Series already.
You're also right that explanations can't be isolated. I'd go further and say they never are. People only come to understand new things in terms of things they already understand. There's no "explaining X" in isolation, there's only "explaining X in terms of Y." A good explanation either supplies a suitable Y or makes it clear to the reader what Y they'll be relying on.
So, even if the author tries to isolate, a successful explanation always relies on collateral information. The only question is whether that collateral information is left to chance or intentionally exploited.
What's the "Y" here? Well, the author assumes the reader can make sense of phrases like "energy at a frequency", what it means to "spin your frequency around a circle", "average a bunch of points along a path", etc.
That's...a lot. I'd go so far as to say that a reader who can make sense of that probably already understands more fundamental concepts like vector spaces, bases, and Taylor Series.
And if they understand those things then there are much more straightforward explanations available!