If we're making a distinction between "exceptions" and "errors as return values", then that implies that exceptions are not return values. And so the question to ask to identify each one is: is the error treated the same as a returned value would be? IOW, if it shows up in the usual return type location in a function signature, and if calling this function plops the value into my lap the same as it would for any other value, then it's errors-as-values. Whether or not stack unwinding is used and whether or not a stack trace is provided is an implementation detail.
Suppose we had a version of the ? operator that automatically appended a call stack to the error value returned. Are you saying that that's not "an exception" because I still need to write ? after each falliable function? Or because it's still part of the return type? Or is it specifically only an exception if it works via stack unwinding?