No it is not nonsense,it happened to me!!!
VFR and IFR have nothing to do with it.
Pop up traffic happened because the climbing aircraft came out of a radar shadow. Just happened to have been blocked from our visual iew due to aircaft structure.
I know you profess to be a very experienced pilot (as per your previous posts) and have lots of experience flying with TCAS, and other technology, but you do show a complete lack of knowledge on some fronts.......Makes one wonder if Walter Mitty has entered the room....