4chan was always kind of the "home" of Pepe memes, so it was kind of natural that the racists, incels, and other "alt-right" denizens of 4chan made (and still make) extensive use of Pepe and consider it "their own" to some extent. But the whole thing about specifically making Pepe into an alt-right symbol was just some shit they stirred up to "troll" the media. It worked, and the stigma stuck.

A lot of Pepe memes also ended up becoming Twitch emotes through BetterTwitchTV and FrankerFaceZ. And a lot of those emotes found their way into Discord chatrooms. In Discord, you can usually tell whether someone is an alt-right person or not by what Pepe emotes they use.