No, the Antlion fight was definitely with Varshahn, Zero and Y'shtola with the castle in the background. The new PvP map is outside Forgotten Springs; that's different.
And I don't think that first phase is Zeromus EG; I think that's regular Zeromus, it's just that our classical image of Zeromus is side-on with a profile shot of its face, while here we were seeing it front-on with the head bowed. It's a little easier to see the resemblance if you compare the DS version.
Incidentally, I always wondered why they replaced Zeromus' design for the western release, and only now did I notice that oh yeah, that's got, like. Entrails coming out of it, yeah, 90s Nintendo of America wouldn't have loved that.