If you're coming from another programming language, sure, you know what ternary operators are already and can reason out what the function does without necessarily knowing Ruby that well, but read the logic out loud:

Ternary: "If instance variable foo equals symbol bar, then return symbol foo_matches, else return nil"

Ruby Idiom: "Return symbol foo_matches if instance variable foo equals symbol bar"

To me, I don't see it as saving characters as much as the ternary version feeling less idiomatic. In the ternary version you've got to convert 5 sigils into meaning in your head to reason out that method, plus the ? and : sigils as ternary operators are less frequently used in Ruby anyways, so you may have to pause a bit longer on them since the ? and colon are frequently used for query-style methods and symbols respectively. That adds context as a requirement to interpreting them in your head. The idiomatic version is functionally equivalent, but reads a little better converted to English. You've only got to convert 4 programming language sigils into meaning and there's no ambiguity of the colon sigil and the ? goes away entirely.