So OpenBMC is fine (happy for them!), but having open firmware is much deeper and broader than that: yes, it's the service processor (in contrast to the BMC which is a closed part on Dell machines) -- but it's also the root-of-trust and (especially) the host CPU itself. We at Oxide have open source software from first instruction out of the AMD PSP; I elaborated more on our approach in my OSFC 2022 talk.[0]
Dell uses trusted platform modules (TPM). It's a separate chipset than the BMC chipset.
For a mostly open source solution, not only would you need open source BMC firmware, you must have an open source UEFI/BIOS/boot firmware like CoreBoot, LinuxBoot, Oreboot, Uboot, etc.
[0] https://www.osfc.io/2022/talks/i-have-come-to-bury-the-bios-...