"Painting with a broad brush" is not an idiom, since it's meaning is deducible from its content, not only its contex. It's a metaphor (literally false as written, but calls to mind an analogy with truth -- both the comment and broad brushes apply the same thing to many targets without any careful differentiation)
http://learnersdictionary.com/qa/Idioms-metaphors-similes-an....
Re:Google Buys Fitbit for $2.1B
by braythwayt on Friday November 01, 2019 @08:06PM
(#21424970)
Great link, thank you, I will use these words with more rigor in future. I think that “metaphor” is a much better word for the thing that is used to illustrate or add emotional oomph to an idea rather than to argue its merits.
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"Painting with a broad brush" probably began as an analogy, but once enough people use it, it becomes an idiom. If understanding is shared between speaker/writer and listener/reader, idioms are a useful shorthand for sharing some commonly understood argument.
So useful, in fact, that people often forget what the original analogy was. For example, the verb "rewind." How many people using it think of reel-to-reels, cassettes, or videotapes?