I actually agree with the theory that Gojo did this on purpose. It fixes some glaring plot holes, and is on brand for Gojo as well.
Basically, from the spectators reactions, it seems that Yuji, choso and Inumaki didn't say shit about Sukuna barrier less Domain to Gojo.
That's like stupidity at the highest level and basically a huge plot hole. A whole month and they couldn't say " Sukuna has barrier less domain. You're not on his level" to Gojo?
But, the theory is that their reactions were for the fans, and some of the unaware spectators.
And that they did tell Gojo. He understood the implications of it. That his Domain was inferior and he would get fucked if the decisive moment of the battle came down to a clash of Domains.
So, Gojo being Gojo, and having the sex eyes that makes understanding and replicating cursed energy usage and techniques easy,
Decided to use Domain at the start. In hopes of understanding how Sukuna does it and being able to imitate it to achieve his own barrier less Domain.
It's much better to do this at the start of the battle when they're both at full power and hp than say, in the middle or even the end. Cause at that point, he would be worn out from the battle.
So chances of Sukuna just Murking him is high.
By using Domain at the start and forcing Sukuna to do the same, Gojo can analyse the barrier less domain technique at full power, and also have CE to escape the domain and heal using RCE.
The fact that Gojo initiated the Domain first seems to allude to this. At least based on panel.
Or Gege is "done" done with the series and is wrapping up lol.