If someone invents a machine that does some necessary consumer-level work (say, a washing machine) and the result is not increased leisure, something is Very Wrong. Sure, we could spend the time we save doing dishes on productive labor, but this wasn't time we owed to the workforce in the first place.

"Increased activity occupies surplus generated by increased efficiency" works for corporations, which is why economists like it, but not for humans, which is GP's point. In fact, GDP is only of practical utility at all insofar as that equation does not hold, since leisure and autonomy and generally not-being-beholden-to-work is the economic end of "human welfare."

There's other points, which I think GP was referencing in a less clear way, about how GDP measures all sorts of economic "activity" that is useless altogether- financial speculation in bubbles, competing advertisement campaigns, etc. But the main point is that GDP is a measure of output, not welfare, and for humans (all else being equal) we're better off the less that we're required to do.