This is a very good article and I agree of much of what he is saying here. However...you knew this was coming, right?!...flax fiber is not nearly as effective for thorough colon cleansing as psyllium is...and for some people flax does little for improving colon functioning. If a person is well-hydrated to begin with and uses sufficient water during the cleanse, the solubility of psyllium is not a problem...and in fact provides greater bulk which gives the bowel muscle something to work against and strengthens peristalsis. Ground flax is a good fiber source for daily use...and also provides some essential oils...although some do better with regular use of psyllium at a smaller dose than what is suggested for the colon cleanse.
He is correct about magnesium hydroxide pulling water into the bowel...but remember that it is pulling that water from the body! If a person is slightly dehydrated to begin with...and most people are!...then regularly using magnesium hydroxide (or salts...epsom, sea, etc.) will further dehydrate the body and increase the possibility of constipation.
Something to keep in mind...he is affiliated with ReNew products which use flax as their fiber source. Most Colon Cleanses on the market use psyllium so this is one way to set them apart from others.
Renew Your Life...written by Brenda Watson (founder of ReNew products and also president of the International Association for Colon Hydrotherapy), Susan Stockton and Dr. Smith...is an excellent book that discusses digestion and detoxification.