The government should use already established OSS. It's not agile to make a document reader from scratch, to increase the credibility of your work, when it's already created.
I don't think anyone is arguing for that. There are literally thousands of services govt is required to provide that do not currently have an OSS analogue - and yet there are proprietary solutions for them. We argue that taxpayer money should only go to fund development of code that is already or will be reased under a free and open license.