To put this nonsense to rest once and for all, the orchestral score of the 3rd says cadenza twice- both in the first half and the second half of the extended solo passage. If you are telling me that you think Rachmaninoff was implying that the recapitulation of the 2nd subject is optional (and that the performer is free to just run a few arpeggios, instead of organically returning to this significant melody) I have no idea who you think you are fooling. You cannot apply archaically specific definitions from a particular era to music in general. Today, cadenza can mean either a solo passage in a concerto, or it can mean a showy solo filler passage in any piece (frequently written in grace notes and without bar-lines).
Can we now return to the subject?