(Kenn White points out on Twitter that some of this may be due to grandfathering --- though, the FedRAMP DNSSEC requirement is pretty old.)
When the DOD tried to mandate Ada, lots of projects were bid as Ada, then switched to C++ at the very first sign of any trouble whatsoever. I would 100% believe it if someone told me that this horrible rollout could be leveraged into an exemption from needing DNSSEC
Was it a hard requirement? No, but the fat fingered audit companies really like to tick that "should" box green and would be more lenient with other debatable findings, so it was suddenly "in our best interests" to comply.