The GitHub page has a section for this:

Exempted Code

Not all source code, documentation and metadata used in the development of Direct File is included in this repository. Specifically, any code or data that is considered Personally Identifiable Information (PII), Federal Tax Information (FTI), Sensitive But Unclassified (SBU), or source code developed for National Security Systems (NSS), as defined in 40 U.S.C. § 11103, is exempt. Due to these restrictions, certain pieces of functionality have been removed or rewritten.