Obviously, TL has to be "what is generally available", and this means that people can and will carry items from other planets with higher TL. They may also sell them there. There is nothing fuzzy or mysterious about this. Say you have a TL 15 world with 10.000 pop with law level 0. The rules suggest you can buy an FGMP if you want. But most of it will be wilderness. Just because it is TL 15 doesn't mean you can buy an FGMP when you are standing on a mountain top 5000 kms from the nearest shop. Or maybe you an order it on Amazon and a little drone will come and bring it to you. The referee needs to decide - does the shop have it? Do they do deliveries? Do they deliver to mountain tops? It is not violating the rules to say no; it is also not violating the rules to say yes.
It goes the same on the next planet over which has Tech 0. The referee can say that you can order Tech 15 stuff delivered from the planet next door; it takes 2-3 weeks at least to arrive, and costs a lot extra because that the the schedule for the liner that comes through the system. Or the referee can say you can't get it, because there is no shipping service, or because there are restrictions against bringing in high tech stuff, or whatever the referee wants to say that makes the game interesting, or makes sense to the referee and fits with the rest of the context.
There is nothing "hard coded" about TL. It also means something different depending on the worlds other stats. A pop 3 TL 15 world just has TL 15 stuff, and maybe some ability to maintain it, but basically imports it all, but is called TL 15 because that it the tech of the stuff that the world population uses. If it is a research station , it might do TL 15 research but only in a very specific area. A pop A TL 15 world also manufactures, develops new products, researchers at the cutting edge of science - probably all sciences. On a TL 15 research station, you might find TL 15 research equipment - probably you can't buy it but it will be there and maybe you can use it if you ask nicely - maybe you can order delivered since they probably do that with regular supply ships. But you won't find every kind of thing: just items that make sense in the context. It might actually be easier to buy a particular TL 15 item at a low tech trade nexus world than at a TL15 outpost.
There are rules related to TL: use them when they make sense and fit your objectives. Ignore or modify them when not. There is no way for Mongoose to make a rule that fits every contingency, so the assumption is that the referee will use the TL concept intelligently.