Pinning too much authority on common transliteration of such radically different langagues as Japanese and English is a bit tricky. The kanji used for the '-to' in tanto (短
刀)is also used in the kanji for naginata (薙
刀) which is certainly even more different from what would be called a "sword" in english than a "knife" is.
I believe there are also multiple different names for rice (gohan, meshi, genmai, etc.) in Japanese, too. Different names do not mean that they are not all rice, though.
Perhaps, but there can be a lot of similarities, too, expecially when they come from the same technological and cultural source.
A katana and a tanto share a lot of structural similarities, but that is due as much to common technology and style issues as anything. The European Rapier and Main-Gauche are probably as similar in constriction and style to each other as the katana and tanto are, yet they were used in
totally different ways. I don't think anyone would argue that the Main-Gauche was a
sword simply based on its structural similarites to the Rapier.
I claim no scholarly expertice on the Japanese language (nihongo) or the history of Japanese weapons, but I have spent 20+ years studying the Japanese martial arts, including time with both tanto and katana techniques, and they are as radically different from each other as those of western techniques for a knife and a sword.