I was not at this event. It was in NYC and it was NYC people who thought that overalls were racist. (Everyone commenting on the post doesn't think it is racist though.)
It was a swing dance event, which celebrates the dance called the Lindy Hop which originated in Harlem in the 1920s but is now mostly danced by white kids with a vintage fetish (some of which are also SJW types who want to make sure you 'check your privilege' and 'don't use heternormative language that can trigger people' etc.).
And the dance itself is possibly named after a known racist and white supremacist.