Imgur was originally offered as a "gift" to Digg users, and when it didn't gain traction there MrGrimm re-branded it as a "gift" to Redditors.
He made a second Reddit post the next day.
Source: https://www.reddit.com/r/IAmA/comments/9tlwi/im_the_imgur_gu...
> I posted it on Reddit, Digg, and a couple forums that I frequent at the same time.
Imgur was never a reddit exclusive thing. It was just mentioned to advertise and gain loyalty from reddit users.
Which, honestly isn't a bad thing if he did.