If that were true, it'd be really easy to detect an Imperius by examining the subject's memories...

Lucius is an Occlumens, and lesser Death Eaters might self-obliviate whole weeks/months of their lives and then claim that they were both Imperiused and obliviated.

Instead, McGonagall's statement implies that the best way to figure out whether the subject was Imperiused is to see if they remember being Imperiused, even with all the information that would allow you to perform Test Foo.

I suppose that throughout the duration of Imperius its victim may be thinking to himself "Damn this Imperius which is making my body do things that I don't want to do", so one wouldn't need to obliviate just the moment of the Imperius, but the whole sequence of events. Which would be counterproductive in Hermione's case as her own memories is the chief incriminating factor.