[swores]

It's not saying the violin IS "merely an analogy for how a double base works", just that a violin can be used as a simple analogy to somebody who understands how a violin works but doesn't know what a double bass is.

Comparing similar things is literally what an analogy is, the fact that in these two cases (radio/light and string instruments) the things being compared are very similar it doesn't make them the same thing, nor does it make it not an analogy.