If the overall system economy is only based on 'ports', ignoring settlements and installations your statement looks correct.
It's not so much that it ignores them, as that their contribution is really small and generally not significant unless you have a
lot of them.
System economy is the two largest components of the weighted sum of all station economies. So, say you have:
- a Coriolis, economy size 15,000, Colony
- an Outpost, economy size 5,000, Industrial,
- three Settlements, economy size 1,000 each, Military
your economy type will be Colony/Industrial for the system (the Military only adds up to 3000, which is third)
If you then build an influencer nearby and the Coriolis becomes 50:50 Colony/High-Tech then that's 7500 Colony + 7500 High-Tech and you'll get a system economy of High-Tech / Colony (High-Tech wins because the influencer will also have its own small HT economy)
There are systems - especially the lower population ones - where the sum of all Odyssey settlements of a particular type is actually the most significant contributor to the system economy. You can see a lot of Agricultural systems on the fringes of the old bubble with populations < 100k like this (whereas pre-Odyssey, Agri systems tended to only be 100M+ population around an ELW or similar) because they've got tens of Agri settlements overwhelming whatever tiny Extraction/Refinery outpost they previously had was.