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.