17 USC § 105 says that any work of a US government employee is not subject to copyright. The OFL-1.1 license, by contrast, depends on the assertion of copyright for its effect. Therefore, there is no way for the work of US gov’t employees to be released under OFL-1.1. (I take no position on whether the terms of OFL-1.1 have been correctly applied here, though I’m skeptical.)