Yes, the majority of the transcriptions were at one time improvisations. Cziffra had made a bunch of tapes of himself improvising when he was quite young (I dunno, 20 or 30 or something) and at some point later in his life, his son Cziffra Jr. suggested that they listen to the tapes and write the music down. At first, Cziffra didn't want to go with it but they worked steadily and progressed over a couple of years (or maybe just months, I'm not sure) by slowing the tapes down to ridiculously slow speeds and then copying down each note by ear, including the chords and clusters, and while staying faithful to the rythms and cross-ryhtms. After the pieces were written down, Cziffra went in and editted some to clean up any loose ends, and then sent them to be published as his transcriptions. In the process, I imagine he also *composed* some of the transcriptions as well, but I don't know enough to say specifically which ones he improvised and which ones he didn't. Most likely, he had improvised them several times before writing them down, sort of like how Horowitz would play the Carmen Variations in concert without having it physically written down and each time was a little different.
Though I am 85-90% certain that his William Tell Overture came about that way. I KNOW that's how his Roumanian Fantasy was born, and if you've ever seen the score/heard that you know it's quite a tricky and complex one.