It is actually that. If you are including an svg using <img> then you can no longer, say, change the stroke color. It also does not inherit your css from the rest of your site even on first load. Your svg is treated like any other regular png or such.
Please review my earlier comment. “Styling properties of SVG elements via CSS” is, on reflection, slightly ambiguous, but largely refers to static styling, which you can do anywhere. If you’re talking about changing things, at runtime, you’re talking about the SVG being interactive, not stylable. And heritability is, as I remarked, another different thing, which doesn’t work for <iframe>, so that can’t be what it’s talking about.
