As historical background information, MMTk was initially implemented in Java, and used on the JikesRVM VM, one of the very first bootstraped Java implementations.

https://www.jikesrvm.org/UserGuide/MMTk/index.html

Eventually they rewrote it in Rust,

https://www.mmtk.io/

Thus it is much more than the 5 years mentioned on the post, that only covers the rewrite.