> the same job could be done with an isolation transformer

It really cannot -- the isolation transformer doesn't have control of its output, so it can't start or stop cleanly, and it can't ramp voltage cleanly. (An autotransformer kind of can, but it's still not really good enough.) The AC source can stop on a dime, with no inductance of its own, so it is the correct way to do this test.

Source: I have had to do this and refused to use the autotransformer anymore because it was just too much of a pain in the butt. (We rented the AC source.)