They don't "support unlimited immigration", they reject the legitimacy of national borders and of immigration as a concept.
For example here's the DSA explaining their view that the national border and immigration statuses are capitalist and imperialist tools to divide the working class: https://www.dsausa.org/blog/fighting-the-security-state-at-t...
I'm still not sure that's representative of the far left. Like I said, the more right wing libertarian position is probably the same, though for different reasons.