Racket is a fringe language. Github has about as many Prolog repositories as Racket repositories. If Racket syntax highlighting was causing performance issues that were noticeable to Github, performance must have really sucked. Why should Github let Racket drag down its capacity?
Is that the message an open-source vendor wants to send to developers: your code is welcome, unless it's a new and unproven "fringe" language? How would a "fringe" developer feel about that vendor's brand after their language has become successful? Would they like/trust/recommend the vendor to upcoming developers?
Reading their thread, it sure sounds like they want to work with the community to make Racket code viewing a good experience.