it's been a term of art for a long time

That's technically true. Indeed, "isomorphism" has been a term of art for about 200 years.

More recently though — over the past decade or so — JavaScript enthusiasts have been using this word to describe code sharing, which isn't quite right.

It's been used to describe code that does A on server and B on client side, where A and B are deeply related but definitely not the same. It's not just code sharing, that's just a small part of it.

My argument is that it's a forced and silly misuse of the word.

I'm struggling to understand also how it came to be [ab]used in this context. To take an old Greek mathematics word and use it to mean something that it doesn't really mean? Why? Isn't that silly? Isn't it pretentious?

Yeah, that's why everyone switched to "universal" after a year or two. But some people are not as up to date and some libraries kept their old names.

I wasn't aware that the favoured term had changed. I do think universal is clearer and much less silly.