The bogosity of clan tartans has been documented since at least 1940; see the footnotes in the chapter on Highland culture in Hobsbawm and Ranger’s The Invention of Tradition (1983).
According to the article, since The Glasgow Herald article in 1895. It turns out that Father Christmas isn't real either, but some things people really don't want to interrogate.
I mention this because I think that the whole idea of political "invention of tradition," while it has lost currency because it seems to reflect poorly on anti-colonial nationalist movements, is far more interesting than that of "random people making up stories for gain."