Thinking about international things within the context of legal/illegal doesn't really make sense. Twitter should take it down obviously. But the legality of another country threatening the citizen of another, comes down to how far the citizen's country is will to go to retaliate.
As an example, the US government does a lot of "illegal" things internationally. But the US isn't beholden to the ICC and no one is strong enough or willing to enforce anything on the US. (Not saying the government should be doing that stuff or that 'might makes right'. Just realistically there's legally zero consequences until that situation changes).
How is this legal.
Thinking about international things within the context of legal/illegal doesn't really make sense. Twitter should take it down obviously. But the legality of another country threatening the citizen of another, comes down to how far the citizen's country is will to go to retaliate. As an example, the US government does a lot of "illegal" things internationally. But the US isn't beholden to the ICC and no one is strong enough or willing to enforce anything on the US. (Not saying the government should be doing that stuff or that 'might makes right'. Just realistically there's legally zero consequences until that situation changes).