turns into a Fire Arrow. Whatever the reason might be that Sukuna chose that particular hand sign, it is clear that he took a page out of Gojo's book. It could be due to ease of use or possibly ...

This might just hint at Ryomen Sukuna's fate and, in totality, the outcome of this battle. In Jujutsu Kaisen chapter 264, Yuji uses a hand sign to activate his Domain Expansion, as is the practice.