I can't disagree more. From thefreedictionary.com:
(Mathematics) A one-to-one correspondence between the elements of two sets such
that the result of an operation on elements of one set corresponds to the result
of the analogous operation on their images in the other set.
This seems exactly like what the author is describing. Colloquial English does not have an objective standard -- it is a constantly evolving language. I certainly agree with this term's usage unless a better one is suggested.
But notice that the isomorphism is the correspondence itself. When you read "isomorphic JavaScript," you would be right to ask "isomorphic to what?" What is JavaScript in correspondence to?
If there is an isomorphism here, it is between the separate runtimes (client and server), and not the language.
