It depends where you find it. If it's a comment, it's highly unlikely it would include careful punctuation such as semicolons, whereas for em-dash you need to do something extra as it's not available on the keyboard as a single keystroke by default, so everybody is using a hyphen instead of em-dash or en-dash.
However, a magazine article, or even a blog where the author cares might include all: printer quotes instead of straight ones, en/em dashes, ellipsis as as single character and many more. If suddenly half of the web is filled with shallow content dressed up in certain styling, people are right to feel something is not right.
> whereas for em-dash you need to do something extra
OPT+SHIFT+- on macOS. It's no more difficult to type than a lot of other punctuation/common symbols.
OK, that macOS. On Windows you had to remember the arcane Numpad combination (provided you had a numeric keyboard). That makes it uneven - the hyphen is just universal.
And on iOS it’s a long-press on the hyphen. It’s not inconvenient at all when you’re used to using them.