That's pretty naive.

This isn't just a simple http proxy and checking if the requested URL exists on disc. It's a pretty subtle snapshotting system that fully understands and vets apt repos, detecting when the repo changes, proactively downloading "interesting" packages that have updated, verifying and validating all packages, and then promoting the repo to active when it is healthy. Plus a web-based admin interface.

The core of it is around 20K LOC, with another 40K in tests. The COCOMO model gives that an estimate of around 2 developer years of effort assuming full time work.