It started as a prank, but now fascists unironically use the ok sign.

It's a dog whistle. They use something innocent to signal to others in their ingroup, and call people stupid when they call them out on it.

I used to believe the "shitpost that got out of hand" excuse too. Let me tell you about something I witnessed a few weeks after that story first blew up:

I was watching an Overwatch esports stream because I still played back then and Blizzard was bribing us players to inflate viewer counts. It was a home game for the Dallas Fuel, so the match was taking place in Texas. Unsurprisingly, this meant the vast majority of the in-person audience was young white gamerbros with a conservative aesthetic.

It also happened to take place on International Women's Day, so between rounds they would have one of the women who worked on Overwatch give a short speech or interview. These were generally focused on their experiences as a woman (and often racial minority), the value of diversity and tolerance, etc. I remember one of the people they brought in to speak was Anjali Bhimani, an American of Indian descent who voiced the character Symmetra.

Every single time they announced one of these presentations, a large number of audience members (Remember: white gamerbros in Dallas, Texas) would immediately raise one arm, make the OK sign with their hand, and wave it around rapidly while frowning. I had never seen anything like it, and given the context it was obvious what they were doing it for.

Blizzard banned use of the gesture during esports matches a few days later. The subreddit was predictably full of posts like yours, downplaying what had happened and ridiculing the ban as an overreaction to a stupid prank. Maybe it really was just a prank at the start, and I don't know if they're still doing it now, but there was definitely a time when fascists were using the OK sign as a dogwhistle and relying on the "media fell for a 4chan prank" story for plausible deniability.