In the words of Mencken, "There is always an easy solution to every human problem–neat, plausible, and wrong."
While it forms the basis of many bike fit methods and I'd agree it's a constraint, knees do not
dictate the saddle position completely. Why would it? Your knee isn't part of the saddle! See
http://sheldonbrown.com/kops.html for another view but surely you've noticed that how far back you want your bum varies with the counterbalancing needed as you lean forwards more? If there was One True Saddle Position for you, you'd be either sitting on the saddle nose on upright bikes (ow!) or flopping over the back on full-speed road bikes (chafe away!). This may be more obvious for riders who often switch between completely different styles of bike.
There are limits and note I was
assuming that the bike fitter put the handlebars in a sensible position for an athletic fit (based on the description and attempt to recommend a road bike) which may not be the case and so it may all not work because I'd agree you can't completely vary the position to reduce the weight on the hands.