Indeed, but it also doesn’t maintain locality of access nearly as well for young objects (the most commonly manipulated ones) and even older ones that survive.
The guts of BTreeMap's memory management code is here: https://github.com/rust-lang/rust/blob/master/src/liballoc/c.... (warning: it is some of the most gnarly rust code I've ever come across, very dense, complex, and heavy on raw pointers. this is not a criticism at all, just in terms of readability). Anecdotally I've had very good results using BTreeMap in my own projects.
In terms of how the "global" allocator impacts performance, I'd expect it to play a bigger role in terms of Strings (I mean, it's a chat program), and possibly in terms of how the async code "desugars" in storing futures and callbacks on the heap (just guessing, I'm not an expert on the rust async internals).
In the current context, fragmentation refers more to the problem of consuming extra memory through fragmentation, which malloc implementations like the one Go (or Rust, or glibc) uses can often mitigate.
