Uh I gotta stop you there man. (it'll be long) It isn't his ultimate attack. That's a false translation. Even Lightning did the same mistake as tcb in calling it ultimate . It is simply just final secret technique.
極ノ番- (Goku No Ban) extreme/maximum/utmost technique
最終奥義- means final secret technique. It's legit an advanced technique a practitioner knows, that is often kept secret and/or reserved for critical moments. Like we're kinda told even in that chapter too or before. If it was his maximum CT then it wouldn't need so many conditions and restrictions and be so ass without these extra help.
極ノ番 is reaching the maximum or peak of an ability, it surpasses the previous to reach a certain lebel of proficiency, while 最終奥義 emphasizes the finality and secrecy of the technique. It suggests that it's the last resort or the most advanced move in a practitioner's arsenal, often reserved for critical situations.
Tldr: don't call fire arrow ultimate technique.
Infact we can even use a obvious parallel here (if you're still not convinced in the fact that fire arrow isn't a ultimate technique) that is blatantly showcased in the narrative: Gojo and Sukuna are similar, very similar. One represents good and the other bad. One has humanity whilst the strongest and the other does not have his humanity whilst strongest. Both have this narrative surrounding them with this "Absolute loneliness from being the strongest, and love", showcasing their parallel. They're kinda like Yin and Yang in a regard.
Now the point of why I brought this up, is because both of these two have three moves, with their final move (the third) always being related to their previous two in order to pull off the third.
With their third move also never being referred to as a maximum technique throughout the entire Manga and fanbook. (Both Hollow Purple and Fire arrow never get called as such in the fanbook)
Hence why, when Kenjaku explains maximum CT, the flashback scene never shows HP and Fire arrow despite their very famous usage if it and are both events before Kenjaku's explanation of Goku No Ban (Maximum technique), and instead the flashback showed Eso's maximum technique and Jogo's maximum technique. (Beginning of chapter 134)