The turbo doesn’t have to start charging at all; it can be bypassed, even. It’s not like a supercharger that’s belt-driven or chain driven or operated by the engine via other means.
S/C’s can be bypassed too. Besides, with a clutch of some type on the front, they can easily be not driven at all, if desired.
If the turbos on these cars were bypassed to fudge the test in a certain rpm range, owners would immediately notice and complain. Like I said before, the days of turbo lag are long gone. Today there is seemless, smooth power from just above idle to close to redline.
Anyway, there are dozens of car magazines (if anyone reads those anymore) and hundreds of car-specific websites that will tell a prospective owner what milage can be realistically expected.
Everyone knows that the “officially” stated figures were derived from some test under lab conditions and may (will!) vary from the real world. All that those figures are useful for at best (if one really cares at all) is as a relative measure between different cars.