Oh yikes. I've never even heard of the term "initialism" before.

I guess that's because it's barely used:

https://books.google.com/ngrams/graph?content=acronym%2Cinit...

Maybe some people have tried to push it as a new word in common usage but they don't appear to have succeeded.

Yeah, I kind of figured that out as you responded - see my other comment hehe. For what it's worth, Merriam Webster seems to agree that the distinction is basically arbitrary, and OED has the more general definition first lol