They noticed a slight rise in turbidity even after ammonium ran out, but speculated it could be due to color change that cells go through (from reddish brown to dark brown) when fixing nitrogen. The small amount of nitrogen in the gas flow was enough to get cells to produce nitrogenase, but not enough for them to use it. But cell-free extracts didn't show different absorbance for the two kinds of cells, despite the visible difference.
When they added chloramphenicol, an antibiotic that inhibits protein synthesis, obviously this inhibited nitrogenase formation. If the enzyme was already present, in vitro, the antibiotic didn't inhibit it. But it did inhibit it in cells, possibly because ammonium built up with no way to use it, repressing nitrogenase.
They tried adding 150 mg N (as ammonium) per liter to a culture of nitrogen-fixing cells, and saw that nitrogenase activity dropped off within about 3 hours. Not as fast as I would expect. They interpreted this to mean that the enzyme is not inhibited immediately, just diluted out as the cells stop producing it while continuing to multiply; but it seems to be inactivated faster than just by dilution, so there might be some inactivation or degradation going on.