I can understand the reasoning, but I would have weighed the significant benefit over the little “complexity”/content increase.

The color inversion is a significant effect. It doesn’t change anything for those that use their own error pages, but significantly improves the situation for people who land on these pages and are bothered by light mode.

/edit: Their PR close comment was super short (non-telling), but they later commented with some reasonable reasoning that better describes their point of view and considerations.