Perhaps that is what it said, but features in the original post like writing goals and document management (in core) go way beyond integration with/export to existing apps and reproduces some of their core functionality. A major part of the reason for using Ulysses is writing goals and managing documents within a project!
Even Obsidian, which in some ways could be better suited to longform writing, has no apparent plans to introduce functionality like that to its core.
If people want to write plugins, great!