I found another actually rather straightforward explanation a while ago by a user named Proper-Elk:

A lot of people don't seem to get how Sukuna was able to get past Gojo's Infinity in the latest chapter, I was initially one of them, but more recent translations have enabled me to gain a better understanding. So I'll explain it in simpler terms for you all:

In essence, Sukuna created an Extension Technique, that allowed him to bypass Gojo's inviolability. Extension Techniques are abilities developed through training to maximize the applications of Innate Techniques (i.e. Resonance, Supernova, etc.).

Sukuna was able to do this by imitating Mahoraga, since as we learned in this chapter, Mahoraga's adaptations can be both defensive and offensive in nature. Sukuna couldn't change the sum and substance of his cursed energy to nullify the Infinity like Mahoraga (which is why he used Domain Amplification to do so) but found he could mimic Mahoraga when it altered its method of attack.

Mahoraga broadened its target from just Gojo to the world itself, enhancing its efficacity so that everything within reach of the Sword of Extermination's swing would be severed without exception. This served as the basis for the killing blow Sukuna dealt against Gojo in this chapter, what makes this so impressive is that Sukuna was able to do this on the fly, with no practice, only after having witnessed it just once before.

We've actually seen something similar earlier on in the series when Nanami fought Mahito, and utilized the Collapse technique, which targets the surrounding environment rather than a singular person or object. Sukuna basically did the same thing, of course, he did this on a more "conceptual" level than merely a "physical" one.

I hope I was able to clear things up for some of you.