Also, at least to me, it feels like there's a difference between being
addressed as "dude" and being
described as "dude".
I wouldn't mind if a friend of mine told me "Dude, look over there!" if they wanted to show me something. It would feel very different if a friend whom I told about being non-binary and being not a man™ described me to somebody else as "the dude over there".
Also I'd like to note that pronouns are not gender, so while most people who prefer they/them are non-binary (and that includes genders underneath the non-binary umbrella), not all of them are. So while I take Sam's usage as a fairly sure sign of Emmett preferring these pronouns, we can't be sure about their gender identity unless they or Jeph tell us about it
And the way I know this comic it might be a while until that happens, if at all, because this comic seems to make a point about normalising self identification and not making a big deal out of it, which I really appreciate.