This is not true at all, and neither is the comment you responded to.
The constitution doesn't apply to a location or a medium, it applies to an actor: the US government (and state/local subdivisions). The US government has to follow it everywhere, and nobody else has to follow it anywhere.
My comment was a bit tongue-in-cheek, as I think the parent was as well.
Americans do tend to run around quoting their first amendment rights like the whole world has them. As you say, it's strictly a US government thing.
While Australia for the most part enjoys free speech, it is not enshrined in the (AU) constitution. The government will occasionally order censorship[1], usually around whistle-blowing, investigations and court cases.
There was a protest in the major Australian papers last year about the erosion of press freedoms[2].
The constitution doesn't apply to a location or a medium, it applies to an actor: the US government (and state/local subdivisions). The US government has to follow it everywhere, and nobody else has to follow it anywhere.