They are calling stuff from the 70's "vintage" now and in a way, that is an accurate term. Vintage means from a particular era or period in time or from a specific batch--think wine.
Thrift stores have all kinds of stuff so I don't know why you would not expect to find vintage items in there. They have brand new stuff with the tags still on it, entire estates of household stuff as well as the usual junk. Estate sales are where many people find their fabulous "vintage" Featherweights and boxes full of fabric so if the relatives don't want a walk through estate sale--they often just dump the entire estate off at their local thrift shop. In some areas, Goodwill & the Salvation Army even have big trucks that will come and pick it all up.
Same way with flea markets now. I was shocked to find booth after booth filled with brand new merchandise. I thought flea markets were always used and/or antique stuff until we went exploring around Fort Lauderdale many years ago. I can't even remember seeing any used or antique stuff at a couple of the flea markets we went to down there.