Seems like they're preparing a new part of Time Temple.

No, it is not open. The game specifically says it's not open.

Source?
Because what it seems like is that the door is open, and you can get in, but when you get in, it's a possible future that this guy made and is showing you, mainly to scare the living bejeezus out of you and demoralize you (incidentally, also making you stronger).
This future still could happen, you must realize. It's one of many possible futures.