Using that definition, censoring all publications of a given group (say, black people) would not be censorship. That's a big hole in the definition.
It's not censorship, it's discrimination. Bad thing, but different. If it was censored because these people write about specific racial issues, it'd be censorship. If it's blanket denial regardless of the content, it's racial discrimination, not censorship. It doesn't mean it's not bad - one could argue it's even worse since it doesn't leave you any chance - you could probably talk in another way to route around censorship but you can't stop being black - but it's not that issue, it's a different one.
The definition you use seems to be a legal one rather than the usual one.
Indeed, but it's not for lack of trying.
> exclusion is based on the content
The pretext the speech falls short of inclusion in the category of "protected speech" because a different common thread can be drawn across its particulars is simply not convincing.
Censoring speech in the basis of nationality instead of content doesn't making the underlying expression any less censored.
> If that's not a free speech issue, there are no free speech issues.
There are plenty, when exclusion is based on the content. This is not one of them. That's why filing a First Amendment lawsuit against trade embargo wouldn't do you any good.