Well I haven't done the depth of research he has, but I believe a key piece of evidence is that in the initial Cybertruck announcement Musk said the Cybertruck

Well I haven't done the depth of research he has, but I believe a key piece of evidence is that in the initial Cybertruck announcement Musk said the Cybertruck uses the same steel as Starship. At that time, Starship prototypes were using 301.