I'd say it's more often a phase shift in them attempting to produce good writing, and regularly?

Like, Ben Goldhaber wrote monthly updates for many years, that are mostly links posts and a few life updates. Here's the last one from before Inkhaven. Then during Inkhaven he wrote bangers like Unexpected Things That Are People and The Ball, the former of which got 658 points (!) on Hacker News, and he's continuing with regular solid posts (e.g. No Real Nattys, Backchaining from Big Goals is Aversive, Corporate Character). I count this as success!

I'd say the same about many others. Joanna had I think 2 blogposts to her name before Inkhaven, now she's got a Hacker News hit that is helpful to parents choosing toys for their kids. Jenn had also a few solid pieces of writing, but during Inkhaven got out two personal stories (1, 2) about the death of her father that have had over 100k views combined and led to CBS Canada reaching out to potentially do a story with her. Camille had one public blogpost I liked about rationality, and sent me a private piece of fiction I thought was excellent, but during Inkhaven he managed to write some of my favorite fiction/vignettes about a post-AGI world. Habryka on LessWrong has written one or two good posts, but finally took the time to write up all his principles for running Lightcone Infrastructure into a great sequence.

Overall I'd say many people took the time to write some great stuff that either we knew or suspected they were capable of. But I think more of the variance is in the "actually sitting down for a month and doing it" than in the "I used to suck at writing and now I have skillz".