I made no comment on the TL of the star port. I did not say you cannot have a low TL and a High TL star port. I said you cannot have a VERY low TL if you have a highly developed star port as a Class A port will give you a DM of +6 on the TL table. Even with a disadvantageous government type the minimum TL will be 5.Within the Third Imperium, Starports are Imperial Territory (managed by the (Imperial) Starport Authority - the SPA). Because of this, you can easily have a low tech world with a Starport with a higher TL than the planet it is on / in orbit of.
See the Mongoose 1st Edition book, "Starports".
I see nothing in Starports to contradict this, but always happy for a citation, every day is a school day.