I mean I don't know how much "better" Twistzz can really get. He was already one of the best players in the world in 2019 (top 5 by Thorin's ranking), and he's a pretty complete player all around. If you set him up for success then he'll play at a really high level most of the time, as he's done so far.
Karrigan isn't known specifically for developing players, he's known for getting the best out of his stars. This means giving them the tools to succeed themselves, because generally superstars know what they need to do once they've been set up. This is a pretty underrated part of being an IGL, and is part of the reason why I think people defending NiKo's IGLing are braindead. He never managed to get a consistently high level out of any of the players on that team, aside from himself and possibly Broky. Compare that to someone like MSL, who brought k0nfig, aizy, Magisk, and Kjaerbye to an insane level, with only Magisk really reaching his peak outside of MSL's leadership. It's also part of the reason why why Aleksib is so great, because on both Ence and OG he's been able to squeeze performance out of players that wouldn't be nearly as good without him.
Part of why Twistzz was never going to reach that level in Liquid again is because no one really cared to help him do that. There's enough publicly available evidence to suggest that he was essentially forced into the backseat both socially and in game on Liquid. What an IGL like Karrigan does is puts stars like Twistzz in positions so they can be the absolute best version of themselves (among other things), which many IGLs don't always understand or know how to do. I don't think his performance was just due to motivation, otherwise he wouldn't have been hit so hard by the player break in 2019.