Blame George Gruhn. Before he opened his first store (right around 1970, iirc), they were all just old guitars. But for Gruhn, “vintage“ was really all about Golden Era instruments from the major US manufactures. He rather quickly was writing for magazines & would tout a ”vintage instrument from Martin’s Golden Era“ & others, and the definition of the word expanded.
Fast forward a bit, and we need look no further than Gruhn’s Guide to Vintage Guitars for the ultimate use of the word. I have volume 2, covering US made instruments made through 1999 - and they all managed to fit into this book under the Vintage umbrella! So yes, nowadays the definition can fly off in a multitude of directions.