In that case the description would be totally misleading, if there is no actual bonus/boost.
"Construct Settlement bonus" it says and "Connect the other three production islands to receive the maximum bonus".
Why would you push players to arrange them in a particular way, advertising there is a bonus, when there is none?
The Blooming Trader Guild boost applies to both, the core and the islands and this seems to be working fine.
In the other linked ticket the set bonus is properly listed as "Set building Bonus".
So there was a boost.
Either it was removed (and the description was not updated) or it "broke" somewhen.