I was wondering about that (whether later samurai had a different primary weapon). If, as per the premise of the article, both opponents had some knowledge about each other (and the knight was wearing plate mail), it seems to me that the samurai would make sure to procure a some type of spear or halberd for the very purpose of being able to get through that armor. For all that the Western world has romanticized the samurai as being brave to a fault, I can't imagine that most real-life samurai would be too proud/stupid to set aside their katana in favor of a weapon more effective against their opponent.