Aha, an argument!!
In a TT you need the maximum power that gets you to the end quickest. That is done in the position set up on the bike by your fitting service.
In training mode on the turbo, you want to train the muscle bundles that are appropriate to the riding position you take on the bike in the real race.
Therefore, if any other position is taken in training on the turbo, muscle bundles are being trained that may not necessarily be used in the race, and muscle bundles that are primarily used in the race are being neglected in training.
In weightlifting, its called “Strict form” where in training, the weightlifter takes as identical as possible movement to lift the bar as would be done in competition.
Oh, we’ve all seen photos of Chris Hoy in the smiths machine shifting big weights. This is good, but Sir Chris went out on the track or sat on the ergometer and trained in his racing position.
The fundamental theory of a turbo trainer is to SIMULATE a bicycle ride. If the training is done for a TT, take the TT riding position and use your imagination as the 25 miles counts up on the distance meter.
If the FTP test is to gauge your power during a TT, also take the TT position. Any other position to get a higher FTP value is cheating yourself.