I believe the author is talking about the OCP (2.0) network card itself, that these adapters internally. The OCP nics are quite cheap compared to pcie - here’s 100GBE for 100!
https://ebay.us/m/HMQAph
What also may not work are Dell rNDC cards. They look like they have OCP 2.0 type 1 connectors, but may not quite fit (please correct me if I'm wrong). They do however have a nice cooling solution, which could be retrofitted to one of the OCP 2.0 cards.
I've also ordered a Chelsio T6225-OCP cards out of curiosity. These should fit in the PX adapter but require a 3rd-party driver on macOS (which then supports jumbo frames, etc.)
What also fits physically is a Broadcom BCM957304M3040C, but there are no drivers on macOS, and I couldn't get the firmware updated on Linux either.
Spec for reference, I’m not 100% sure. https://docs.nvidia.com/nvidia-connectx-5-ethernet-adapter-c...