Maybe Prevost was talking about the requirements to become a classical musician with a safe orchestral job in which case he might have the glimmer of a point. Otherwise the comment as reported seems to be a bit, well, tosh. Quite a few of the great jazz musicians had classical training of some description (Oscar Peterson's 'lineage' (student of a student of...) goes back to Liszt and therefore to Beethoven as well). Various jazz musicians have made classical recordings (Goodman, Jarrett...). Many of the best composers in any age have also been regular improvisers (Bach, Mozart, Beethoven, Liszt, Messiaen, Barrett!).

But it's a well-known trap to bag someone out for the report of a report of a comment, isn't it? Never mind, I've typed it now.