How does this manage to say that ZGC and genZGC are not production ready - “not that ZGC and genZGC are production-ready” - then later reference JEP 439 and note that Generational ZGC is production ready with, “At the time of writing, Shenandoah Generational is experimental, whereas Generational ZGC is production-ready” (production-ready is a link to the JEP)… then go back to “On a side note, Oracle's ZGC Generational—GenZGC—is not supported in production yet.”

Is this a separation between RH OpenJDK (which, being OpenJDK, has all of the JDK 21 features including JEP 439 in it) and something special Oracle is doing, or?

The presentation of the two and difference discussion is quite confusing, even knowing something about the JVM, for something styled as a beginner’s guide. If making comparisons it’d help if they were clear, especially when promising a “straightforward” look at the start and directing people who want detail off to “upstream documentation”.