In the video below, which struck me as very interesting, especially his remarks on his brother Christopher Hitchens.
The amusing meme that the American neoconservatives who planned the Iraq war (which Christopher Hitchens supported so vehemently) were the new liberal Trotskyists is a favourite of libertarians (see here), and there may well be an element of truth to it, given that the neoconservative foreign policy was a radical departure from the realist school of foreign policy that had previously been influential in the US government. But let us move on to my main point.
Marxism has been divided into the (1) internationalist wing and (2) the nationalist wing. What Peter Hitchens says here can be applied to the internationalist wing, but not necessarily to the nationalist communists/Marxists.
After all, it is not difficult to extract an anti-open borders ideology from what Marx wrote about mass immigration, and the Soviet Union had highly illiberal immigration and emigration policies (see Light 2012).
In fact, mass emigration was such a threat to Communist states that they actually had to resort massive intervention from keeping their populations from fleeing (e.g., the Berlin Wall).
But it seems to me that so many modern Marxists these days have just been infected with the same Postmodernist rot, cultural relativism and pro-open borders ideology that informs so much of the mainstream left, and this is another source of why Marxists support open borders.
Light, Matthew A. 2012. “What Does It Mean to Control Migration? Soviet Mobility Policies in Comparative Perspective,” Law & Social Inquiry 37.2: 395–429.