Agreed.
I just hesitate to call that "Unicode string" an "alt text". For me, an alt text is something in HTML (or XML): The content of some tag's "alt" attribute.
To my excuse: Precise terminology can prevent confusion. Example: "Plain text cannot contain alt texts." - "🤔 Wait! The alt texts of emojis are read out all right!" Or: "The document author determines the language and content of alt texts."
At the end of the day, we all agree what authors should do.