You’re right that it is an initialism, but you’ve got the inclusivity backwards. Initialisms are acronyms that are not pronounceable, but they are still acronyms whether or not they are pronounced. You did get me questioning my own memory, so I looked it up to double check:
Sometimes, initialism or alphabetism is used to refer to acronyms formed from the string of initials which are usually pronounced as individual letters, as in the FBI (Federal Bureau of Investigation).
If it were all of them, then the main wiki entry for acronym would not be allowed to stay the way it is. The main article I linked even speaks to the fact that some users of “initialism” think that it is separate from acronyms, so there is definitely still some significant debate. And to add to it, I looked up the words in the three most popular dictionaries:
Oxford dictionary lists both initialisms and pronounceable abbreviations as two separate definitions of acronym, so according to them, all initialisms are a form of acronym, matching my inclusivity.
Cambridge dictionary has acronym and initialism listed as unconnected entities with separate definitions that do not mention one another, so there is no confirmation of inclusivity either way there.
Merriam-Webster dictionary defines initialism as any first letter abbreviation, acknowledges the debate, and claims that initialism is the older word, but it also says that pronounceable initialisms are commonly referred to as acronyms, so their definition more lines up better to your inclusivity rather than mine.
So it seems like possibly one or both of us is right or neither of us is. Isn’t the ambiguity of the English language fun?