Is CSS for SVG “standard” CSS?
No, it is not. It makes use of the CSS language, but it is not in the CSS property specification. It's only part of the SVG specification. It's quite similar to using non-standard vendor extensions, in that while prefixes are defined in the CSS grammar, prefixed properties don't actually validate as CSS solely because they're non-standard.
You're supposed to choose the SVG validation profile when validating your code, but it doesn't appear to work; it spits just as many errors as validating according to the CSS spec would. In that case, then it's probably a validator bug.