Could you link the Samurai Archives page that says he didn't exist? The only SA references to Keiji that I could find said that he was only a relatively minor member of the Maeda clan whose modern reputation has been exaggerated by popular culture (i.e., video games).

Anyway, he certainly seems to have existed. He's listed in all the Japanese reference books I checked, although usually under the name Toshioki rather than Toshimasu (Keiji had a number of different names over the years.)