I always find this moral duality quite interesting, the product only exists because of the sponsor.
No, it not because of "the" sponsor, but because of sponsoring in general. Replace the sponsor by any other company/institution/patreon/....
You seem to be unfamiliar with the situation, then. The sponsor wasn't for NixOS itself, but a related in-person event. NixOS had existed for literal decades prior to this.
I highly doubt he would reject DARPA as a sponsor for NixOS. This was just celebrated by a bunch of safety people that have significant overlap with NixOS security goals: https://www.darpa.mil/program/translating-all-c-to-rust