> These are mostly narrow pre-emoji symbols that have been extended with an alternate emoji representation.

Nitpick: this is incorrect. Easy counter-examples would be arrow symbols like →. UAX #11 helpfully explains what is "ambiguous" about those characters:

Ambiguous characters occur in East Asian legacy character sets as wide characters, but as narrow (i.e., normal-width) characters in non–East Asian usage. (Examples are the basic Greek and Cyrillic alphabet found in East Asian character sets, but also some of the mathematical symbols.) Private-use characters are considered ambiguous by default, because additional information is required to know whether they should be treated as wide or narrow.

In the other words, these characters have been commonly available in both Asian and non-Asian character sets and assigned different widths by them.

There are preexisting narrow symbols that were given a new emoji presentation in later standards rather than assigning a new codepoint. Text rendering engines vary on which form is the default. VTE had an option to set the preference. This can be very annoying when some arrows get the new emoji form but others in their cohort stay as narrow glyphs.