What They Saw
They were looking at cell extracts, protein activity in vitro. Nitrogenase can reduce azide to ammonia (and other stuff); carbon monoxide can inhibit this process completely, and nitrous oxide (N2O) partially. More ammonia is formed from azide than from dinitrogen even. However, each azide molecule is reduced to one N2 and one NH3, so two thirds of the nitrogen becomes gas instead of being fixed. Presumably if left long enough, all the N2 would become ammonia though.
In terms of amount of hydrogen produced with azide present, it varied from between 28 and 35% of what was produced with no substrate present, similar to dinitrogen.
With cyanide, the enzyme reduces it to methane, ammonia, and methylamine (CH3NH2). CO inhibits this reaction completely, and azide and nitrous oxide partially. When nitrogen gas and cyanide are present, less base is formed than when only nitrogen is present, so there is competition.
In terms of hydrogen again, with cyanide there was 17% as much hydrogen as with no substrate, but it also seemed like cyanide reduced the electron flux to the enzyme overall, to about 30% of when fixing nitrogen. If CO was added with cyanide, it restored hydrogen production up to 58% of what it was with neither. This is pretty consistent with previous results.
They also tried methylamine reduction directly, but it was a very poor substrate. The same was true of cyanate (NCO-), and they didn't detect any reduction of CO to methane. (Though they maybe should've looked for ethylene, since the Mo nitrogenase produces mostly that and none of methane from CO).
What This Means
So nitrogenase can reduce a bunch of things pretty well: nitrogen, nitrous oxide, azide, cyanide, and acetylene. It's an interesting enzyme.
This study has a good summary table of different substrates for nitrogenase, products made from them, and their effects on hydrogen production/electron flux.