I think you are a tad too generous in your grading criteria! I would have awarded a "9," but not the perfect "10."
I would say that you are "basically correct," -- with respect to Koi ponds. The bacteria -- performing nitrification -- do deserve the credit for converting ammonia into less toxic forms. From everything I can determine, there is not appreciable off-gassing of ammonia at the pH conditions habitable to Koi.
For the sake of scientific accuracy, however, your above statement about ammonia off-gassing is not entirely true. A major issue is pH: In water, ammonia exists in a state of equilibrium between two forms: actual ammonia [NH3] and ammonium [NH4+]. In a nutshell, one can off-gas the ammonia form, but not the ammonium form. The relationship is pH dependent: at higher pH, there is more "ammonia" form, at lower pH, there is more of the "ammonium" form. At the pH appropriate for fish ponds, however, most the ammonia/ammonium present is in the non-off-gassing "ammonium" form. This is why showers -- when used in Koi ponds -- really do not appreciably off-gas ammonia.
A different situation, however, may be observed within the waste-water treatment industry, where "ammonia stripping" is practiced. Bearing in mind the issue regarding pH, in ammonia stripping, the pH of the effluent water is raised significantly. This high pH (like 11-12) drives the ammonia/ammonium to the "ammonia" [NH3] side of the equilibrium. The NH3 then can be significantly off-gassed. Tall towers are often implemented to keep the ammonia gas above the breathing zone of adjacent buildings (lest the area reek of ammonia odors). I have a technical paper in PDF form at work -- if your are really interested, send me a PM and I'll email you the document.