I read it a long time ago 
At a certain point, you stop 'practicing', and the time you spend alone at the piano becomes 'rehearsal'.
I can grasp this quite well mentally. It might be in part true for me already, but at which point it would be entirely true I can't say. If I had the opportunity for regular, rhythmic practice ... a few weeks? A few months? A year? A decade? A lifetime? I don't know. So, for me it's like knowing the earth is round instead of flat, but not yet having the exact evidence to prove it in the way I think I ultimately could, or that others currently can.
My current concept tells me that it is possible -even logical- that learning brand new pieces can be a rehearsal, with the right applied principles and concepts in place, even if a massive repertoire is not already in place (which, in my case is not (having started formal training when I did, etc)). But, I'd of course like the opportunities to discover my higher abilities in experience and manifestation, vs. in theory only.
Reading Whiteside so far gives me permission to be a rhythmic learner, perhaps the one that I naturally am, and that I believe I am on the deepest levels. I have felt it necessary to squash this aspect of myself for many reasons. I won't go much further into it, other than to say that there is a very deep, rhythmic learning cycle that I can feel within me as a vital aspect of my being, but which I have never been able to experience in full. I have known parts, but almost never the whole. And, it is, I think, this particular aspect of training that I believe others who have been formally trained in "all the right ways" have had, and have no idea what it's like to live without that, can't actually even begin to truly comprehend what it means for that to be missing, and who may very well take it for granted. Perhaps I am wrong, but there is not much within my experience that would evidence that I am. All I can hope is that whatever my learning cycle truly is, that it is (quite a bit) bigger than what I can perceive, and that it is actually taking place right now, and that there will at least be times when this is clear.