"Off the rails" is a
weasel word (or rather, weasel phrase) in that it doesn't actually define what you're talking about. It just vaguely alludes to the idea of "metaplot has always been bad" without establishing how or why that is.
Given that they've apparently been seeding the obelisk thing in adventures for years on end, are referring to the events of previous adventures (e.g.
Rime of the Frostmaiden), and are potentially about to reference a major event from AD&D 2nd Edition, it looks more like they're about to tear the coffin lid off.