If measuring tension and comparing to a published spec it must be done without tires inflated. The reason is very simple if you take a moment to think about it.

If tire inflation causes the tension to change, than the amount of change would vary according to the pressure. That means a manufacturer cannot publish a tension spec with tires inflated unless he also specified the inflation pressure at which the measurement is to me made.

There's also the practical consideration of requiring a wheel builder to mount and inflate a tire in order to check tension, then to remove it when done, or to work on the wheel more.

It's analogous to your height. You measure it without shoes on otherwise the measurement would change daily depending on which shoes you wore, this would mean that a woman would have to give her her height as a range that varied through a range of 5 inches or so.