Yes, that at sign (@) is a convention used on Stack Overflow to address other people. The developers have taken that "convention" and made it a functional direct-reply mechanism.
But the convention started in other social media.
The first place where it became the social norm, I believe, was in IRC [citation needed :) ]. Participants would use the
@ symbol to refer to other folks in the discussion by their nickname.
I believe the convention was adapted from e-mail, where @ was used to mark a passage as "attention" to someone (other than the main recipient) in the cc: list. If I was sending a message to Jeff about Waffle Wednesday, I might also cc my assistant, Marc. The message would look something like this:
I ordered sixteen dozen waffles for the office. They should be here by 10am.
@Marc, please confirm.
@ symbol had no functional purpose in email. It was simply convention.