I'll respond in few ways:

1. The EARLIEST, UNCONTESTABLE, PROVEN reference to a belted plaid is in 1594. Any earlier is a matter of debate and highly and hotly debated.
2. IF you want to argue ealier, MANY, MANY WILL AGREE WITH YOU.

3. It was called "that ancient garment" by the late 1600s (I read it, but forgot source, so can't quote) and was highly believed to be very, very old by those in the 1700s and 1800s.
4. Many, today, follow and believe this line of arguement. Also, many want to believe this and part of me still does too.
(see number 2)

5. MANY don't care regardless.