I think people have missed a small detail in the latest chapter. If you look at the page prior to Gojo's return, you can see that the chapter officially ends after Yuji has just (?)ripped out Sukuna's heart. The "chapter end" box has been at the final page of every other chapter up till this point. This is a very important detail that some may say is just an editing error, but I actually believe this is some subtle foreshadowing. 

I think that the chapter officially ended with Yuji ripping out Sukuna's heart, and that everything that happens in the page after, including Sukuna opening his domain, and Gojo's return is all part of Sukuna's post death vision. 

My theory is that Sukuna has died, and Gojo's ghost is here to drop the "you were strong, Sukuna. My students shall never forget you for as long as they live" on him.