Lol well teaching this professionally surely makes me some form of authority (albeit of course not the authority!) on this subject.

To clarify, your original point sounded like you were making a distinction between metaphorical mirrors and actual mirrors:

“in the mirror” tends to more often refer to a metaphorical “mirror”, typically when discussing self-reflection

• “I took a look in the mirror and decided to change my ways.”

“in a mirror” tends to refer most often to actual mirrors that exist in reality, not metaphorically

• “I looked into a mirror to fix my eyeliner.”

This incorrect distinction is what I was objecting to, because of course we can use both the indefinite and definite articles to refer to either literal or figurative mirrors.