Fossil is a source code management system first and foremost, not forum software. Therefore, the `/timeline.rss` feature that the RSS feed depends upon does not give you forum posts by default.
I believe the URL you want for this is
https://sqlite.org/forum/timeline.rss?y=f
The key is the parameter at the end.
You can read more about this [in the online docs](https://fossil-scm.org/home/help?cmd=/timeline.rss).