This mailing list article is about providing new or modified behavior in the kernel to guarantee behaviors - and security - to userland programs in any language.
Re: Disabling multithreading, I think you might be referring to disabling symmetric multiprocessing (SMP), also known as hyperthreading.
Java's virtual machine is so substantially different it's hard to know where to begin.
OpenBSD is a kernel and a general purpose operating system. It runs arbitrary userland software.
Rewriting the kernel in Java will not make C user code safer, and the cost to convert OpenBSD is so nontrivial as to make it an absurd suggestion. I mean, really truly absurd. As in, "am I even talking to real people who work with software?" absurd. If you think it's a good idea to rewrite a posix-compatible OS kernel in Java, by all means, go do so and make a patch against OpenBSD. It's a monumental undertaking, and I'm sure it'll be impressive!
On the other hand, rewriting userland software in Java (from C) will definitely make it safer, but users can already do this and are choosing not to. The goal of this change is to make it easier to write safer C.
I am fully aware UNIX clones will never use anything beyond C for most part, although there are some famous UNIXes like Apollo that did indeed trail another path.
Even if the kernel were written in Java - which is, again, absurd to ask of the OpenBSD developers - it would still be a viable syscall to add for user mode programs written in memory unsafe languages.
Symmetric multiprocessing (SMP) is when all processors have uniform distance from all memory, as opposed to non-uniform memory access (NUMA).
"Eric Bier Demonstrates Cedar"
https://www.youtube.com/watch?v=z_dt7NG38V4
Or, for another example,
https://people.inf.ethz.ch/wirth/ProjectOberon/Sources/Kerne...
It is only incompatible with UNIX school of thought.
I mean, there's so much work to keep C safe and make sure memory is managed well, etc.
What the project needs is a virtual machine! Clearly they don't care about performance when security is on the line (they did disable multithreading, no?). A good virtual machine will be a small performance loss compared to dealing with this stuff... forever.
Of course, you still have to deal with user land binaries.