I don’t think you’re missing anything, I’m seeing the same limitation on the forum side.
The two workarounds that seem to work right now are:
1. use the search box in the right sidebar for broad keyword lookups
2. use Google with a site filter, for example: site:projectlibre.com/forum/discussions baseline calendar or site:projectlibre.com/topic critical path
For me the Google route is a lot faster because it surfaces old threads and exact phrases better than browsing category by category. It would definitely help if the forum had a visible full-text search box at the top of each discussion page.