Happy New Year, Talon! Yes, you're right, "matte" and "flat" are synonymous.
I'm not sure of the chemical makeup of Testors Dullcote, but I use it with acrylic paints, and it's just fine. With it and other similar sprays, it often takes more than one coat before things look truly matte / flat.
As for Testors Dullcote, you may be able to order it from the www.testors.com
website, tho' it may be a bit pricey ordered from there.
Best of luck, and happy customizing!
... the Findswoman