Interesting OS. Its 'VP code' looks like a precursor to Java bytecode/HotSpot, but much more low-level and RISC-ey.
Inferno OS's 'Dis' VM took a similar to approach to VP code, if I understand correctly.
I presume that, in 1991 when the article was written, "JIT" wasn't yet in the techies' parlance. It's not used anywhere in the article.