The rust compiler makes use of custom arenas for allocation, quite heavily in fact. And does it without using the nightly-only custom allocator alloc types. Instead, there are functions that let you build structures inside the arena, plus a bunch of macro logic that builds on it. And while rustc generally uses a lot of nightly features, there is nothing fundamental about it that requires nightly.

Also, again, it's a fair concern that you want to be doing custom allocators, but this is not the same as claiming that no-std applications can't use the heap at all, which is what the blog post did. For simple heap usage, a global allocator is enough.