Would a non Sikh MP coming into the building be disallowed from wearing one? Is a knife or sword of that size considered in law to be an offensive weapon? If the answer to either of these questions is affirmative, then they should not be allowed to wear them. Ceremonial or religious Kirpan can just as easily be made from wood or white metal that doesn't take or hold an edge. If they are made from steel that can take and hold an edge, they are weapons.
Unless Mr, Ignatieff can quote extant legislation that allows the wearing of a Kirpan, I believe he is wrong.