Reading Bookworms quote I would interpret as
1 - If your flight is permitted by this convention or other special authorization to overfly a state then you are allowed to overfly the state.
2 - If you are going to stop in a State (or are compelled by their overflight rules to stop) the State can require that your first stop in and last stop out are at Customs airports and the State must make the list of Customs airports available.
I can see no reason why a technical stop on an international flight should be allowed to use a domestic airport. The whole logic of an international port is that there is a part that is 'outside the country' and a line which is then 'inside the country' as long as you stay 'outside' you don't need to pay import duties, have Visas, ditch goods that are prohibited in that country. Landing at some random internal location defeats the whole logic - how would you argue you hadn't 'entered the country'??
States can always make rules that say 'If you are from XX then it is OK to enter somewhere other than a Customs Port'. And of course Emergencies are different - you need to receive permission in triplicate before you can crash
