Only part of the GGC fleet are Dell machines (that pdf lists Dell, HP, and Equus). Paraphrasing one of the leads from some years back: "Single-vendor is not a vendor strategy."
Between improved negotiating position and resilience to vendor-specific firmware bugs / vulnerabilities, the additional maintenance cost associated with supporting two or more platforms pays for itself very quickly.
In this particular case, they're the air-gapped product is singly dependent on HPE servers, mostly for compliance reasons. Same reason on why it uses Palo Alto firewalls.
Though in the case of the GGC nodes, having multiple vendors was mostly a negotiating component. If we could go to HO and order 3000 servers and have them running, Dell loses a large amount of negotiating power.
Being honest though, working with Dell was significantly better than working with HP or (especially) Equus.
Between improved negotiating position and resilience to vendor-specific firmware bugs / vulnerabilities, the additional maintenance cost associated with supporting two or more platforms pays for itself very quickly.