The rest of the stuff, incl. I/O is actually on the trivial side - threads do require planning. This is what I meant by being a 'toy' project, threads (and JMM) would be impossible to bolt in later on.
But, as the previous commenter attempted to point out to you this project is a *self-described* toy project.
On the very page that is linked, the author of the JVM specifically says:
"I want to stress that this is a toy JVM, built for learning purposes and not a serious implementation."
Thus, absolutely no one disagrees that its a toy JVM. They just want you to stop being dismissive of someone's toy project by repeatedly pointing out its a toy project and not a "not a thing"
SQL (ACID) over multiple non-cache-coherent nodes is extremely difficult to pull with regards to consistency, though.
Thats... why it's a toy! I'm really not sure what you're missing here.