The government purchases $100 billion of goods, issues $100 billion of bonds, and raises taxes by $3 billion a year in order to amortize the bonds. Government purchases go up by $100 billion this year. Private consumption goes down by $3 billion this year. Net fiscal impetus is not $0 but rather $97 billion. Cochrane doesn't understand the Ricardian Equivalence argument he is trying to make.
Now, ignore the Ricardian Equivalence, but just note that Keynesians think $97B was created via this spending and taxes this year, which is why they love government spending regardless of what it is spent upon. But if they issued $100B in bonds to create this 'new demand', where would that $100B have have gone otherwise? I don't think this makes sense in general equilibrium; money is never 'idle' unless it is under your mattress. I guess if you could caricature the two views: one thinks supply creates its own demand, the other that demand creates its own supply.