According to the guy who effectively kickstarted the emo scene in DC, (even if he wasn't really the first or not), Ian McKaye, hardcore and emo are the same thing to him. Why? Because emo is about raw deep expression of emotion. This is how a lot of hardcore punks would describe hardcore music. Though over time emo as a thing developed a more different range of sounds. It wasn't until bands like Jawbreaker, American Football, and Sunny Day Real Estate got sorta-popular to where punks went like, "oh i guess its not really the same thing and it is stylistically different enough"