The definition of "antique" is somewhat vague and subject to social evolution.
As a general term-of-art these days, and "antique" is any item over 25 years old. So all these Antique Stores, and the "Antique Roadshows," even antique cars, etc. are all dealing with items 25 years or older.
Now, the legal definition of antique, as applied to firearms, is different.
So it is not inaccurate to refer to these Lend Lease M1's as "antiques" and may actually be clarifying for the general populace of the U.S.
Personally, I hate it. Antiques are, to me, 100+.
Peace favor your sword,
Kirk