I think there are a few other pieces of info that weigh against V's timing being a retcon, and that's ignoring the fact that there's been no hard evidence that it was ever presented in the future. (That one thread seems to just have someone speculating that it
could have happened 50 years from now, not that it
did.) Within the whole crazy meta-thing that the Uru timeline has become, all of the Myst games were created based on documents found during the DRC's excavation of D'ni. From an IC perspective, Myst V exists as a real game, a depiction of the actions of whoever freed the Bahro, which we later learned was Dr. Watson's doing. As such, since the game was released in 2005, the events it depicted had to have happened before that...and since we were flat-out told that Dr. Watson was the one responsible, the only place it could fit was between Prologue and MOUL.
Here's another fun tidbit: I purchased the special edition of Myst V, which came with a copy of the official strategy guide (in near-microscopic print, no less!). The guide is written from the perspective of one of the Explorers who had participated in the events of Prologue, and explicitly mentions the Restoration having ended previously, which implies that it was meant to be set soon after Prologue from the beginning. Now, granted, this isn't in-game canon, and guides aren't necessarily completely accurate: the official Myst and Riven guides were written from the perspective of someone in the present day, but given that Uru didn't even exist then, it's not really surprising that they wouldn't mesh with what came later. (Plus the whole "fictional games" thing again.) By Myst V, Cyan had the whole Uru storyline well in-hand, so I think one can assume that what's mentioned in there holds up.
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Prepare for Descent...