You can put a URL into an SMS message, but you can't put an anchor tag (link) which is comprised of [a hidden URL and visible link text]. The latter is the only one of these that benefits from the symbol in question, because the external nature of the hidden URL is disguised. The former doesn't benefit because the external nature of the URL is obvious.
Modern SMS readers will linkify the URL, but will set the visible text equal to the URL, so again it wouldn't benefit from the symbol.
This explanation mirrors the rejection rationale: you don't need a symbol to be a Unicode character if the document is already rich text; the symbol can be an image instead. If you have anchor tags, you probably also have image tags or CSS.
> This explanation mirrors the rejection rationale: you don't need a symbol to be a Unicode character if the document is already rich text;
No, that's not the rejection rationale. The rejection rationale isn't about having access to image tags or CSS. It's about hypertext (click link, go to another text) vs having text (can't click link, no other text). If that was the rationale, they wouldn't allow emojis in unicode, because you could insert them as images.
Sorry, let me rephrase in the way that I did in another comment: it seems that to be a codepoint, it needs to be useful in plain text scenarios.
Emoji are useful (arguably I guess) in plain text scenarios because they exist to convey additional information about an author's emotion, and that author might use plain text. The external link symbol is not useful in plain text scenarios because it exists to convey additional information about an author's preceeding hypertext, and that author isn't using hypertext.
Modern SMS readers will linkify the URL, but will set the visible text equal to the URL, so again it wouldn't benefit from the symbol.
This explanation mirrors the rejection rationale: you don't need a symbol to be a Unicode character if the document is already rich text; the symbol can be an image instead. If you have anchor tags, you probably also have image tags or CSS.