That's not really what's happening in the article. If you read through the single file benchmark, you'll see several clever algorithmic improvements (like rarest byte guessing, building a set of variants for Unicode-aware multiple pattern matching, etc...).

The author literally concedes that the .gitignore feature was not done for performance, and actually carries a significant overhead in large directory trees. For the sake of comparability, the study was controlled for the .gitignore overhead.