TL-11 tech is, and it is entirely sufficient to build interstellar workhorses like the Type S, the Type A and many others.
Yes, they can be manufactured at TL-11, or even TL-9, but they are not necessarily identical to what the Imperium produced millennia ago.
Higher tech can have gone into the design of those ships and their components. A jump drive C manufactured at TL-9 can achieve jump 6 which requires TL-15 knowledge. The Ziru Sirka certainly couldn't do that. A jump drive C manufactured a few centuries ago in the 3I couldn't do that.
You seem to envision the OTU as an enlarged version of 20th/21st century Earth, with technology apparently (only, I would argue) advancing rapidly at all times. That is just not the picture Traveller presents. Tech plateaus out, further progress is glacial, and millenia-old starship designs are still valid. You might not think it realistic, but that is the way things are presented.
The history of the 3I is the history of technological progress. The original selling point of the Sylean Federation and the 3I was: Join us and get access to our tech and trade networks.
Entire TL steps takes centuries, but that is just a label of what is considered normal in the 3I at that time. Each step is large and consists of many smaller steps that are continually researched and strived for.
Technological progress is an intentional major point of the 3I, see research stations, and a major mission of the Scout Service.
The 3I isn't static, it was very different 500 years ago or 1000 years ago, both technologically and politically. There are plenty of TL-12 worlds in 1105, but it's generally not the same worlds that were TL-12 a thousand years ago when that was Imperial max TL. Not only the Imperium, but the successful worlds evolve and progress with time.
The 3I is very different from the Ziru Sirka, that was centralised and kept static.