I have a pretty big line of reasoning I was going down in my head, so this may end up being a pretty lengthy post, but it'll either demonstrate Gojo had gotten noticeably stronger since Volume 0, Kenjaku is (for whatever reason) significantly weaker than Geto was, or that both Geto and Gojo are frauds with low IQ. I'm strongly leaning towards the initial conclusion as it seems the most probable, but I'd be interested in other people's opinions.
The first thing which stuck out to me re-reading Volume 0 was Geto's confidence as opposed to Kenjaku's own confidence. When Geto is envisioning a war between jujutsu society and his own group he claims the
likelihood of victory would be below 20% ("呪術連まで出てきたら2割にも満たないだろうね" = "If the jujutsu sorcerers show up, it probably won't even reach 20%") and that, if he possessed Rika, then that would reach 99% ("だがその なけなしの勝率を9割9分まで引き上げる手段が" = "However, there is a way to raise that meager win rate to 99%"). However, when Kenjaku is discussing his plan to defeat Gojo, the conversation is completely different. Kenjaku is
better at curse spirit manipulation than Geto ever was, has such efficient RCT that
he can regrow arms like it's nothing which is
very unlike Geto, and is
one of the few people in history who can match Tengen in barrier techniques, with the added benefit of
10 million curse spirits relative in
quality to Geto's, gravity manipulation and
a potential 4th CT. Kenjaku is also confident that
he could effectively no-diff Yuta, which
everyone in
the verse agrees with unanimously, so gaining Rika also isn't an issue with Kenjaku. So Kenjaku by all means should possess that 20% likelihood, if not the 99%, and yet the fanbook says it would be
IMPOSSIBLE for Kenjaku to kill Gojo under any conditions, which
Kenjaku repeats and even suspects
he'd need 1,000 years of prep time to fight Gojo, with him also saying
Gojo could kill him INSTANTLY and living in fear of that prospect.
My initial interpretation here was that Geto was simply wrong, that he simply didn't know how strong Gojo had become since his teenage years, which is possible given
even Kenjaku had severely underestimated Gojo's strength ironically enough. However the scarce relativity between Geto and Gojo in this time period doesn't seem like just an error; Gojo himself considered
Geto's strategic knowledge almost infallible and when
he was in arms length of Geto he couldn't attack out of fear
Geto would have enough speed to react and harm the students, something Kenjaku absolutely cannot do given
he saw 16F Sukuna and Gojo as a blur when they clashed, and Gojo has the six eyes so overestimating an opponent becomes far less likely. Even unbiased perspectives like
Mei Mei sided with Geto winning the war, so there definitely wasn't a recognised massive gap between them. Further, we then Miguel who's
physically equal to Volume 0 Gojo in certain areas who Geto believed could only stall Gojo (compared to believing he could BEAT Gojo), and with this same Miguel have vastly inferior feats compared to current Gojo. So it could still be the case that both Geto AND Gojo were wrong and that Miguel is secretly a top tier who, due to being cowardly, simply never showed off this strength, it's possible, but is it also not just more likely here that Gojo - the guy who is
known to train his CT even
after he became the peak of jujutsu society - got stronger over the course of a year? We also know Gojo never reached his full potential, during the Sukuna fight he was growing to new heights,
using moves he never even conceived of before. My friend also proposed "maybe Kenjaku is just weaker than Geto", but for reasons already expressed I severely doubt this as probable.