The term "isomorphic JavaScript" also sounds strange to my ears. I think it’s because in traditional use (in mathematics, linguistics, biology, etc.) you don't say "X is isomorphic", you say that "X and Y are isomorphic".

Grammatically, it’s kind of like the word "similar". I would feel silly saying "JavaScript is a similar language", and I feel equally silly saying "JavaScript is an isomorphic language". Similar/isomorphic to what?