Fistly how do you know that they always dressed differently?
Secondly, Dressing differently is not the same thing as having a uniform. (my proof is jewish womens dress today even by the yeshivish is not absolutely uniform.)
Thirdly Yeshivish is about alot more than dress. Outward appearance includes actions being done for what it looks like you are doing to yourself or others.
As for outwardly appearing jewish, nobody thinks my yarmulke means I’m a Catholic minister.