Dressing up as sonic doesn't make you a furry because sonic is an old character that existed when furries weren't that much popular and because it was the old times, and dressing up as him doesn't make you a furry, it can be for reasons like it's for your job, or you're trying to achieve a bit comedy, so dressing up as him doesn't make you a furry just because he's an anthro, but dressing up as your anthro character makes you a furry (obviously) because that character was made to be a furry, and they do actions in that costume that makes them a furry, so thats why.
Also do you know what stream you're in