That is a weird term for two reasons. First, isomorphism is an invertible structure-preserving mapping between two structures. While the trivial case of that mapping being an identity is technically also isomorphism, it renders the relationship between the two structures (here code bases for different environments) into an identity as well. At that point the multiple code bases you're talking about are identical, not just isomorphic. It's like calling humans "vertebrates". While technically correct, if you're talking for example about the consequences of wars, do you talk about the loss of human lives, or the loss of vertebrate lives? I imagine it's not the latter.
And second of course, "portable code" had been the accepted term for this "pattern" (if you can even call it this way) for decades already. I'm not quite sure why one would feel the need to randomly rename things that have already had perfectly functional names.
The word portable is too general to be meaningful here. If I said my JS library was “portable” most would assume it meant that it ran across multiple OSes under Node.JS. If I said it was isomorphic, I need no further context, because it’s a well-understood bit of jargon in that context.
Personally, I don’t really think it’s useful to dissect the semantics of every bit of jargon; plenty of it is semantically imperfect, like the word “factoid” used to describe factual information or figurative uses of literally. It’s just a word being used to describe a somewhat specific concept in the context of a niche. You would still need to be initialized in what it means in context even if it were semantically correct. The helpful thing here is that a simple search like “isomorphic js” gets you up to speed almost immediately. OTOH, if I search for “portable” libraries on NPM, it’s all “cross platform” stuff. If this term didn't exist, that would make it hard to find libraries and frameworks satisfying this niche.
It’s neither here nor there, my chief annoyance with this general thread was people claiming that “this is just like PHP” or “this is just like Rails” and the problem is, it’s not really like that at all. It’s not like older attempts at “live” server code, nor is preact components really much like templating. The different approaches have their merits, and achieve similar end goals, but the developer experience is starkly different in ways that are maybe difficult to understand from simple examples, but absolutely definitive in real world apps. Don’t look at me, though. I write all of my backends in Go.
And second of course, "portable code" had been the accepted term for this "pattern" (if you can even call it this way) for decades already. I'm not quite sure why one would feel the need to randomly rename things that have already had perfectly functional names.