Earlier quoted context omitted.
You clearly didn't read the article. It mentions that in addition to the (relatively small) bundle size savings, the version of Preact they were investigating (as of 2020 when it was written) was also more compatible with the current version of React they were using at the time than the newest version of React (15 -> 16 had a lot of breaking changes). I don't believe the bundle size was really a major contributor to…
Doesn't explain why they could not have continued using React 15.
> While it would be really hard to describe here in detail all of the new features, the code architecture improvements enabled by the React v16.8 Hooks functionality are so significant that it will eventually become harder and harder to recruit developers interested in working in a pre-Hooks codebase. In order to keep up with the rest of the industry and provide our current developers with the best tooling available, I believe it should be a high priority to enable use of Hooks and other modern React functionality. This definitely isn’t anywhere close to the only new feature worth upgrading for, but it does represent an extremely large shift in how developers will be able to write React code.