Cuz we didn't see how he got cut after last chapter and suddenly we see Gojo's body being cut in half ( at least from the spoilers )
thats why people got mad and called it offscreen which in my opinion is true
While i do agree with Sukuna Instantly slashing him, imo we need to see how the "process" of it for its not called Offscreen fight. Just like when Mahito Killed Nanamin and "Nobara". Even the Death of people in Shibuya by sukuna, we see their "reaction" at least before they dead. Mimiko, Nanako, And Haruta too.
Like imagine if Yuta fighting Kenjaku, Yuta all bloodied, seems to be losing and we see Kenjaku Bubble speech claiming he is winning at the end of chapter, and then suddenly next chapter you had a flashback of Kenjaku, and next few pages later you see Kenjaku's body only being half remained. Thats an Offscreen fight imo