If you had read the next sentence, I stated that I confirmed what I saw on flyertalk by finding the same information on Air India’s website. In fact, the information on flyertalk had been copied and pasted from the relevant Air India page. So I did not use flyertalk as my final source of information. I verified what I saw there by going to the airline’s page.
The segment KTM-LXA-CTU was one flight (CA408). For people whose destination was CTU, they were deplaned at LXA to go through immigration procedures, since the continuation of the flight (LXA-CTU) operates as a domestic flight. I’m guessing that from an immigration point of view, they considered CTU my first destination in China, but they just happened to look at and stamp my passport at LXA. The really strange thing is that during that whole process, right after immigration, they had us exit through customs and I was temporarily “present” in Tibet! Our next step was to proceed to a checkin counter which was closed, other than a single agent that stapled an extra document on to our KTM-CTU boarding pass and then we walked around to another section to go through security to re-enter the terminal. Between those 2 stations, I walked by an exit where I could have easily gotten out of the airport and into Tibet, without anyone seeing me. Obviously I wouldn’t have gotten too far since my passport didn’t have the necessary papers, but I still thought it was strange that the setup was so lax given how strict they are about people entering Tibet.