We're talking about two different things.
There's nothing stopping someone from a TL6 world in a universe of ubiquitous interstellar travel, from going off world to a TL15 trade school to learn about starship maintenance, and bringing back a set of tools and equipment to facilitate such work to their TL6 world.
All it takes is money, and not even that much money. 1000's of CRs, not millions, even with bad exchange rates.
Whether there's a market for such expertise on a TL6 world is a different story. Do they really need a tech on world to do this? Isn't this something the engineer can handle?
What they may well need, is parts, and, well, those are perhaps unlikely.
But there's that ubiquitous trade thing again.
Ever popular "O, Brother where art thou" quote.
Pomade Vendor: I can get the part from Bristol. It'll take two weeks, here's your pomade.
Ulysses Everett McGill: Two weeks? That don't do me no good.
Pomade Vendor: Nearest Ford auto man's Bristol.
Ulysses Everett McGill: Hold on, I don't want this pomade. I want Dapper Dan.
Pomade Vendor: I don't carry Dapper Dan, I carry Fop.
Ulysses Everett McGill: Well, I don't want Fop, ▮▮▮▮▮▮n it! I'm a Dapper Dan man!
Pomade Vendor: Watch your language, young feller, this is a public market. Now if you want Dapper Dan, I can order it for you, have it in a couple of weeks.
Ulysses Everett McGill: Well, ain't this place a geographical oddity. Two weeks from everywhere!
And, if you can import parts, you may well import a tech with them at the same time. You have millions of CRs of starship stuck here, dropping a few 10,000s of CR to get it flying again, while painful, isn't unreasonable.
There's an adventure seed. "Tom Stock, the traveling starship mechanic. Have core modulator, will travel."
So, there can be a TL15 trained tech on a TL6 world, it's just unlikely to be a fruitful use of his time and expertise.