You do know that Gojo doesn't need other people's domains to cast his own, right? Right? Gojo called him weak because he is weak to Gojo. It's literally as simple as that.
And I'll repeat what I said above and what we know.
That's their plan. Trap Gojo or stall him. What do you think Geto wanted Sukuna to do? They placed Gojo in an unfavourable position and had to pull out a magic box because nothing else would work, confirmed by the author.
I don't understand what you thought Geto wanted Sukuna to do. He orchestrated an entire point around trapping Gojo with people and then trapping him with Prison Realm
and if the prison realm doesn't work unleash Sukuna in the same environment where they are and continue fighting Gojo while Gojo needs to hold back. And yet, the author decided that the plot box has better chances of working because Gojo is too op.