When the rumours of Nemesis closing for a year to be re-tracked started circulating, Sub-Terra re-opening to compensate for that was also part of that rumour.
The rumour of the re-track was probably circulated deliberately by the park themselves, they would have known that some, maybe even most enthusiasts and fans of the park were probably going to react badly to, arguably, the best ride being closed for an entire season. So to counter that, put it out as a rumour so any bad feelings regarding it could be offset by the fact that it might not actually be happening. Then over time, the more it is spoken about the more people actually get used to the idea, so by the time it is confirmed to be happening the impact of that is lessened.
I think with that in mind, the work going on inside the building, and the fact that it isn't, as far as we know, being used for Scarefest, we can probably assume that the original rumour was correct.