They don't mean immutable as in language form, but immutable packaging system is the general form where you can add but not remove packages to it so as to not b

They don't mean immutable as in language form, but immutable packaging system is the general form where you can add but not remove packages to it so as to not break things, which is the common form among most maven/cargo/hunter/etc.../etc... dependency packaging systems. It's generally considered that npm supporting deleting packages was a major mis-design, which became very public when a popular tiny package got deleted, which then broke so many things, so they learned that the hard way instead of learning from the systems that came before (obviously not cargo, but you get the drift ^.^).