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To me what this means is that he is essentially "trained" at using improvised weapons as his weapons. He is "untrained" with using all other weapons. So its like the opposite effect.
Imagine Jackie Chan in any of his movies using a ladder, bucket, etc. He is very proficient in using unconventional weapons. When it comes to conventional weapons, he takes a -4 (He doesn't like bladed weapons and feels he might cut himself or hurt others with them so he uses them sideways, ie hits with the flat of the blade).
What I feel is that this template essentially flips improvised weapons with normal weapons for the rules. It just provides a base for each type of normal weapon for damage type and amount, ie a light weapon functions as a light hammer, a one-handed weapon functions as a club, and a two-handed weapon functions as a quarterstaff.
As far as flurrying goes, a monk cannot flurry with any weapon he or she is not proficient with. A flurry constitute years of training and knowledge with his weapons which allow him to gain extra attacks. In this case since improvised weapon are his "normal weapons", and normal weapons are "improvised weapons", he cannot flurry with his version of improvised weapons, ie all normal weapons.