Clarkson's ability to coach is perfectly relevant when discussing Lyon's ability as it provides a benchmark against other coaches can be measured.
Clarkson's brilliance or otherwise shouldn't be measured by what other coaches do but by what he is able to do.
As for Lyon's apprentices, well Chris Scott was an ass coach to Mark Harvey and never to Ross Lyon. So, if we take your argument to the conclusion you had before, then we should have kept Mark Harvey as coach as his apprentices have had success, where Lyon's haven't.
Anyway, as I said, judge a coach on what they achieve and not what others achieve