Incidentally, Marie was born in Canada, but has family in Russia, where he was previously under investigation for conspiring with hackers to tamper with a previous election, and is tight with the president. An authoritarian known for exchanging favors, mob connections, and making political dissidents disappear.
So as for the credibility of the threat (Referencing Somalia, not Siberia. Subtle!) She's testifying to the very patterns of behavior, using international connections to "Deal with people" like the family of Joe Biden, that she believes from her personal experience, he uses to gain, and preserve his power.
Ignoring all that, why someone might be intimidated by a tweet. He shouldn't have sent it, nor said anything about a witness testifying against him, because it's illegal, and undermines the proceedings. It's witness tampering, by definition. Obstruction of justice. You can't obstruct justice to beat charges of obstruction of justice. That's like robbing a liquor store to pay the bail for robbing a bank.
It also kinda shows a propensity for obstructing justice.