I have to say that I hate it when people claim things are "pretty much confirmed", when the truth is nothing is actually yet confirmed.

In other words, don't confuse opinion with fact, fan-fiction with actual story events.

As for the shard worlds, we don't know enough about them to confirm anything, beyond what we have learnt about the first and last reflections. One is in danger of falling into a flood of light, while the other became the Void.

Anyway, such topics pop up so regularly, I wonder, again, if it's a good idea to have a sticky message somewhere stating the definitive, official answer: "No, Hydaelyn is its own standalone world, not part of some earlier world featured in other title in the franchise."