This character was code 0x01 in the original IBM PC code page (https://en.wikipedia.org/wiki/Code_page_437), and hence in DOS. It was displayed single-width and monochrome just like any other 8-bit character, never causing any rendering issues, unlike emojis today. It was added to Unicode for round-trip compatibility with that code page.
Emoticons are not the same as emojis. For one they allow for more expression or personal style by having different variants, e.g. :-) vs :) or for absolute maniacs: (:
They are also not limited to what some consortium and a couple of megacorporations think you should be able to express.
I think there's a difference. The code point will always mean "eggplant", it just happens that the concept can be interpreted in different ways according to context—just like the word itself. But ":-)" can only ever mean "colon minus rparens" before further interpretation.
Actually, according to Unicode, "-" doesn't mean minus - U+002D is hyphen-minus.
And as for the eggplant, your semantics-as-specified are useless when 99.9% of the usage has a different intended meaning due to the inherent lack of expressiveness in a corporate-approved emoji language.
What’s it called is syntax, what it’s means is always context dependent. That’s why we invented formal notation, so that we can have context free interpretation (it’s bundled with its semantic so you don’t need to apply some context to it)