Yeah there’s a bit of industry worry about that very eventuality — hence the ultra Ethernet consortium trying to work on open source alternatives to the mellanox/nvidia lock-in.
Cisco have sat on the steering committees for a lot of things where they had a proprietary initial version of something. It's not that unusual, and also, it's often frankly not actually that open; e.g., see the rent seeking racket for access to PCI documentation, or USB-IF actively seeking to prevent open source hardware from existing, etc.
Eh, the UEC effort is a standards org through the Linux Foundation so it won't be subject to any of the usual chicanery. And actually, it looks like Nvidia is jsut a general member and not one of the Steering Committee members;