That's definitely a Gen 2 action. The big rotating crossbolt at the top was a feature of the 1st Gen Nucleus actions like the one I have.
The ARC website has kinda always been a little behind, from what I can tell. If you look at the description of the Gen 2 Nucleus, I'm pretty sure they are still using the text from the Gen 1 description for the bolt stop.
There's a "Nucleus update" thread somewhere on here that describes the changes, pretty sure the bolt stop you have was part of the Gen 2 changes, as opposed to the Gen 1.2 changes.
Edit: Just looked at the website and looks like they are still using pictures of Gen 1 Nucleus for the Gen 2 Nucleus product listing, but use the Gen 2 pictures for the Nucleus Gen 2 & Xylo chassis complete rifle package.