My proof is below.
The first proof is in the transmission itself. There is nothing sealed about it and it is not the proper technical name of the unit. It lacks a dipstick. The fill and drain holes enable the technician to add the required fluid whether just performing a simple drain and refill. 2-3 quarts. Or when fixing / replacing the transmission. Up to 9 quarts. There are detailed technical instructions on how to get the car to temperature, etc. Dropping the pan to replacing the unit, a drain and refill, the fluid goes in at the same place.
Second, the lab tests done on the fluid at around 100k show a reduction of up to 85% of various clutch pack additives, reduced viscosity, and added metal content from wear. In other words, the fluid had about 30% of the original additives left. Also, the clutches wearing is what causes the fluid to dirty or turn color. What helps them wear less is the special additives in the fluid. It's only logical to keep this fluid in ideal condition.
Third, general information known about transmissions and what almost every other Lexus dealer is doing. Why yours is doing this is beyond me. Probably worried about liability alone... Most Lexus dealers do the drain and refill. Some have the very expensive flushing machine.
Call around and you will see. This is the same story on the ls460, BMW, and a few other manufacturers. They can get away with this because by far majority of first owners will not keep the vehicle but only 3-5 years.