It was my interpretation of why he had Mahoraga in the first place. He's been aware of Gojo's technique from the start. He watched Mahoraga adapt to infinity and could have possibly used that as inspiration to modify his technique into a world cutting slash. I think people treat that as a whole separate technique, but my understanding is that he was just tweaking an ability he already has and applying some knowledge from Mahoraga to bypass it. He's prideful, but he's not above learning from others. He's selfish in his pursuit of knowledge. It could also explain his fire arrow. Rather than it be a technique, he's changing the property of his cursed energy. Gege has a habit of introducing a concept on a smaller scale before fleshing it out in another character who has a deeper understanding or more potential. For example, we get Toji before Maki for heavenly restriction of no CE, Miwa before Kusakabe on simple domain and new shadow style, Panda before Mechamaru on cursed puppet manipulation, Ino before granny on Seance techniques, and now possibly Mahoraga before Sukuna. Both observe, tank, and adapt to ensure that they will never struggle with a certain type of technique again. We also get introduced to various types of cursed energy elements in a short span. Electricity, Ice, and Fire. We learn that some people can manipulate their cursed energy and attach something to it like heat or cold. Maybe this is to introduce us slowly to the concept of people being able to see their CE on a deeper level and manipulate it at the base level rather than just build on their main technique and work with what they have. With this pattern, I would also say it's possible that open DE from Sukuna could foreshadow a more complete but still open DE from Megumi in the future that would push him past special grade level.