>Ultimately whether the others get deplatformed doesn't directly affect them, so what's the real difference here?

The difference is -- if I know the effective policy, I can plan better and choose a different platform in advance (if the it's option (b)).

>This seems highly subjective & ultimately not really bad by any metric that matters.

Subjective opinions matter because people act on them (more) consistently than on objective facts. I would for example act on a subjective opinion that a platform provider is a bunch of racist assholes or affiliated with r-country.