Hydrazine (H2NNH2 or N2H4) has a molecular weight of 32.05 g/mol and a density of 1.02g/cm^3
1L of Hydrazine = 31.83 mol
1 mol Hydrazine => 2 mol N + 4 mol H
4 mol H + 2 mol O => 2 mol H2O
31.83 mol Hydrazine => 63.66 mol H2O
63.66 mol H2O has a mass of 1146g or 1.146L at STP
I think I got that right.
While his mental calculations weren't accurate to four decimal places, from a Fermi calculation standpoint he was close enough to the actual value to act on his estimate. He took a bunch of shortcuts but his logic "each hydrazine molecule yields two hydrogen molecules" is sound which means that two liters of liquid hydrogen combined with sufficient oxygen to make water should yield roughly two liters of water, which it actually does within about 10%.
There are certainly other examples of deviation from factual science but this one doesn't seem that bad.