I have no doubt the premise of the article is very true for playing pieces and for physical technique. Practice simply makes permanent, good or bad. But what about improvisation I wonder ? What, if anything, constitutes the "practice" side of improvisation ? Is there any practice side to it at all, and if so what precisely ? When improvising, I invariably play to my strengths, never to my weaknesses, which latter course would be self-destructive, so that alone would seem to put improvisation out of the terms of his discussion.
Jarrett has stated that he does not believe it is possible to "practise" improvisation; you either do it or you do not. I know it is possible, indeed very productive, to practise elements of improvisation in isolation - keyboard harmony, physical technique, figurations, rhythm cells, phrasing; I do it every day. But developing all the ingredients to perfection won't make an edible cake.
The only way out of the dilemma seems to be that perhaps every improvisation over a lifetime, starting from the very first youthful fumblings, is at once practice and performance. Performance in that once done, the result stands immutable, for better or worse, as a finished work of art; practice in that its execution, combined with repeated listenings to it, busily grow the musical psyche and feed future improvisations.
Of course I realise the article isn't about this aspect, but my grasshopper mind saw the title and I asked myself the question.