Autocannons are not machine guns

Of course, but when you say that sentence, you're using the word "car" and intend it to mean something specific. That's its definition; it's what people mean when they say it.

In this case, when people say "cannon", they sometimes mean something that could also be described by the word "autocannon". So in that sense they are synonyms. Therefore, one of the definitions of "cannon" is "autocannon".

If you want to say that one of the definitions of "autocannon" is a "cannon" with certain particular characteristics, then I'm ok with that. I'm not ok with saying that a "cannon" is an "autocannon" because that would be a lie.

I've decided I can live with adding "sometimes used as shorthand for "autocannon", when mounted on aircraft" if it's genuinely important to you. Emphasis placed on shorthand.