Very interesting, and I didn't put it together at the time, but I was explaining to someone the difference between true and truth. In Yiddish the difference would be true = ריכטיג, and truth = אמת. According to this it'll be grammatically incorrect to say "וואס דו זאגסט איז נישט אמת". It has to be either "וואס דו זאגסט איז נישט ריכטיג" or "וואס דו זאגסט איז נישט דער אמת". Saying "וואס דו זאגסט איז נישט אמת" is akin to saying "this is not table" instead of "this is not a table".
In short, truth is a noun, and true is an adjective.