That works right up until you start looking at X-boat routes. These are based on an assumed J4, the X-boat's ability. Many of the systems on them aren't even accessible to a J1 vessel unless it makes multiple jumps.
Next, virtually all X-boat routes within the Imperium stop only at A or B starports so routine maintenance is almost universally available. I would think that the Scout service would make the necessary spares at the appropriate tech level available locally regardless of the local TL, which usually is A+ anyway.
Since the tender can't enter an atmosphere, it would have to be serviced at a high port or yard in space. Easier still, would be to have a support ship with a J2 or 3 capacity that moves along the X-boat route performing periodic maintenance if it is so specialized rather than relying on locals to do it.
All that the tender being TL 15 argues is that when first put in service it could be delivered by a lift ship (it's just 1000 tons itself) to a system that isn't at TL 15 and couldn't manufacture it. The alternative to that would be that the computer, avionics, possibly weapons (again making little sense 98% of the time), and other TL 15 musts are imported while the ship itself is built locally.
What need is there for it to be 100% TL 15?
Also CT supplement 7 pg 46 shows the tender to be TL 10... That is something I would have expect. No need for high tech except the computer, which has to down- and up-load data to the boats.
As for the idea of it being taken or "seized," that seems rather far fetched in most of the systems its being used in. While not universal, many have naval, military, or scout bases in them too. I doubt that some shown as having a route through them are actually stops, like for example, Lleska in the Gushemege (2240) subsector, a red zone with a X starport. I'd think that one is a "flyover" system...
Others would be major ports with lots of space traffic. I doubt that such a system would lack some sort of equivalent of the Coast Guard to come to the rescue. After all, the tender wouldn't be off in some remote corner of the system to begin with.
While a number of obvious stops are amber zones, this is generally because the system is run by a tyrant or three and has law levels well up into the lettered region, not exactly a warm fuzzy to criminals...
I could see design provision for arming them, say in wartime, or the like but not normally having this installed. It would just add cost for no useful return.
Another question...
What sort of ship is used to move data and mail from the tender to the main world of the system?
This would be necessary for stuff like say, diplomatic mail, secure or private mail that the sender / recipient doesn't want transmitted electronically for whatever reason, and for small packages and the like. I'd assume the X-boat system includes something like Fed X or DHL service...