They explain it in the article but I'll paraphrase here.
Historically there wasn't a single good answer/method that could be applied universally to many of the problems. Hence user selectable and configurable plugins to allow the user to experiment for what worked best for them.
That is no longer the case, the project has matured to the point that a optimal choice is known meaning that the existing plugin system only provided the user with a footgun. This they folded plugins and their developers into the codebase and built a database of known optimal configurations.
I wouldn't imagine it was that much, I would suspect the API remains largely unchanged, just the library code gets compiled in rather then being loaded at runtime.
I read your comment as more not seeing the benefit of the change.
Historically there wasn't a single good answer/method that could be applied universally to many of the problems. Hence user selectable and configurable plugins to allow the user to experiment for what worked best for them.
That is no longer the case, the project has matured to the point that a optimal choice is known meaning that the existing plugin system only provided the user with a footgun. This they folded plugins and their developers into the codebase and built a database of known optimal configurations.