Huh, this is surprising to me. Anecdotally, I tried Minecraft Java under Rosetta on my MBA, and it ran smoothly 2560x1600 resolution at 60 FPS with very few drops. Although I'd imagine that a JITed game might be easier for Rosetta to deal with, as it will be running the same code paths over and over again in comparison to a compiler.
The linked article with the benchmarks doesn't make it clear what JVM is being used. Since the Java benchmarks seem to perform well, I assume they're using a native JVM?