I think that's the real issue: timing. The only time this can work is when someone has just resigned or joined a new company, so they can (and are actually willing to) "legitimately" pump up the volume about themselves.
So make it an easy opt-in triggered by these events. Any triplebyte candidate that "closes the deal" should get opted-in automatically. Anybody without an ongoing work relationship, should get opted-in automatically. Everyone else, you hold fire until something significant happens publicly, at which point you gently prod them. You can even ask, when someone signals they are looking for a job, "do you want your profile public at this time? It's a pretty cool thing! If not, no biggie, we'll ask again once things change."
It's not rocket science to do this respectfully and it's sad that they didn't.
Am I misunderstanding you? If you "get opted in automatically", then it's no longer opt-in; it's opt-out.
If they made the initial launch opt-in then that signals that the user deliberately chose to advertise that to the world. The message a current employer gets out of something that's opt-in instead of opt-out is notably different. This is just like the whole opt-out fiasco with the Do Not Track header. If it's opt-out, the signal is largely meaningless. In this case that's a benefit.