They talk about the specific systems in terms of legacy code and how far removed government agencies are from automated testing and other modern, best practices. It has been a couple of years since I read it but I recall a part about a business process at the IRS that that people don't start learning until they have been there for about 17 years - due to the complexity. It talks about how there had been failed attempts to migrate to a new database, some of the data is now duplicated but the upgrade is de-funded so all the new code has to be aware that data
may be duplicated.
I'm not sure if this book got into it but I've also read that the IRS has assembly code from the 1960s that is very optimized and only a few devs can work on it. ChatGPT knows a lot about this history as well.