if he uses broker or clearing house, the employees at the brokerage or clearing house can see his entry and exit, from where they can figure out the possible reasoning of each trade. If I am given simons' trades, I can guess out how medallion works.
Can you really? I highly doubt it. Maybe if you knew what the inputs into their models are as well...but only the trades? You should be able to accomplish the EXACT same thing by looking at the tape...but you cant. https://en.wikipedia.org/wiki/Observability
You/the brokerage/clearing house cannot "guess how Medallion works by looking at Simons' trades." If you could, the strategies would have stopped working.
Does the owners own a brokerage, but most likely much of the trades occur in dark pools with exception of futures. Bet they have so many many automated systems going, be impossible to know whether they getting in or out. Plus, if he is hedging, would further confuse. I don't know if you have to report transaction of dark pools as they happen or same day or next few days?
your reasoning is wrong. tape is full of random information, while his trades are not. his trades are based on non-random observations.
darking pool trades may not be reported to tape immediately, but clearing house has accurate records of the time and price of simons' trades.