I think where we are differing is on whether or not a definition can be extensible and malleable.
Because English is primarily oral, definitions tend to be democratic. Irregardless is an excellent example. Previously, If it was even acknowledged as a word, it was treated as an antonym of regardless. Now, due to common usage, it is treated as an irregular variant.The dictionary does, and must, bow to consensus.
As you've mentioned before, we've seen "Bowie" move from a particular form to a class of forms. Now, though there are common elements to them all, if we want to be specific, we need modifiers. Modern, historic, sandbar, etc. Homo Sapien, Bowie Modernis?
Could we not apply the same classification to schools of art?
And if we do allow for such extension, could we find examples that would, at least, qualify as a subspecies of Dadaism?