If I'm understanding you correctly, they're basically doing the same thing as Python under the hood and using a heap-allocated array (vector) of pointers? If so, that should still be orders of magnitude faster than a linked list.

If their implementation is actually a linked list, colour me shocked. My impression was that JavaScript is "decently fast". I've never even considered writing high-performance code in it, but I've heard that the compiler can optimise extremely aggressively, and it's used so widely that I couldn't imagine that it had glaring performance issues like what I would expect to see if every array was actually a linked list under the hood.