The way I feel the good sophonts of the IISS would interpret that is by logging "Iderati" as 22 billion people, which is still an A pop with an additional 2.
The lower tech Ag world in the system is a pretty simple case of most of the manufacturing being on the mainworld IMO, and thus the lower logged TL. I'd expect a lot of high tech stuff there though - they just dont make it there.
I'm pretty sure the Sol system had something a lot similar in the OTU btw, with several meaningful pop worlds.
From memory from GT:FT, it's a log scale, so I'd say three worlds wouldnt get you a half point - you would need five. That said, if you need routes for plot purposes, handwave it as a "more trade than usual" world, because megacorps X, Y and Z do preventitive maintainence for all sorts of expensive crap for the entire sector there.
In the long run, the problem with lots of little colonies is security - defending one world out to 100 diameters isnt that hard, but defending a number of them from smash-and-grab raids is, or arranging convoys in and out is ... so you only tend to get one mainworld.
Ian