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.