Are you sure about this? What about a brand-new colony with 2,000 colonists and a TL-15 Seed Factory? Is that not a TL-15 world?
No, that is an example of 'high tech items brought in from outside' until it can support itself with locally-produced TL-15 goods. Also note the idea behind the 'colony' label implies that the world is not self supporting.
Just one TL-15 seed factory is going to be a production bottleneck, and there is no redundancy or resilience. Sure, the factory might be
able to produce sticks of colored chalk for the chalkboards at the elementary school, or 9-ply scented toilet-paper, or a hyper-luxury grav-car for the governors girlfriend -- but while it is doing so it is
not producing life support supplies, or repair parts & spares for the powerplant, or mining equipment, or any of the parts required to make other factories.
As a referee, I would tend to rule that a single TL-15 manufacturing facility actually could
not produce all of the above items -- it is not a Fabber, and (if it were) then the Fabber would be producing (at best) TL-13 goods. The colony needs to build the TL-15 Manufacturing Facilities to produce (a very generous minimum of one MF per product) all the above stuff -- ie. "a large network of production facilities, resources extraction industries, physical infrastructure, service industries, and so on...."
[Edit:] This comes back to my contention that the Technology Level of a world depends on the Population. Take your 2000 colonists -- every man, woman, and child -- and grant that each of them is capable of overseeing the automated legions which make up a single Manufacturing Facility. Now compare that to the number of types of goods that a TL-15 society needs to be producing, not just water-filters, CO2 scrubbers, piping, fiber optic cabling, or 'computer chips', but also every item on the shelf at the big-box store, and every component that goes into the vehicles, smart-phones, and etc. 2000 people is certainly not enough; and in this extreme example of 'even the infants work', nobody is teaching the required skills to the next generation of workers, nobody is doing the administrative or management work for the colony, nobody is doing the health-care for anyone else, and so on.
Tech Level requires population. [/Edit]