That may be, but I have yet to read one that was legitimate. So far, all of them have not stated that they have completed the requisite steps. Your report is the same as the rest.
Entering WT3 does not grant the ability to craft NM sigils.
From: https://maxroll.gg/d4/resources/nightmare-dungeons
Unlocking Sigil Crafting
In order to unlock Sigil Crafting at the Occultist, you must first complete a Tier 3 Nightmare Dungeon. Follow these steps:
- First complete a Tier 1 Nightmare Dungeon, which you acquire through the Tree of Whispers in Nightmare Difficulty as noted above.
- As you kill monsters and progress through the Tier 1 Nightmare Dungeon, you are likely to find a Tier 2 Nightmare Sigil. If you do not, run any Tier 1 Nightmare Sigils you acquired in that dungeon to try again. You can also reliably acquire additional Tier 1 Sigils through the Tree of Whispers.
- Run a Tier 2 Nightmare Dungeon or Dungeons to acquire a Tier 3 Nightmare Sigil.
- Once you acquire a Tier 3 Nightmare Sigil, complete that Dungeon to unlock Sigil Crafting!
You have not stated in your post that you have done this, and neither has anyone else who posts here.