I don’t believe it’s a mini-BNC either. I was told by Proficnc (part of CubePilot, I think) on Facebook that the antenna connectors on the Herelink ground unit are custom (i.e., not standardized).
Because of that, if you want to connect an antenna of your choice, you might have to open up the housing of the device to use the internal U.FL connectors that are currently wired to the “custom” (incompatible and rage-inducing) external connectors.