It is not hypocritical or contradictory to believe that deplatforming is appropriate in some cases and not in others.
I agree with GP that the arguments one makes should reflect one's actual reasoning, e.g. "I believe it's wrong to say thing X and therefore I support deplatforming in that instace". But because "I support deplatforming in every instace" sounds stronger many people falsely claimed that to be their reasoning. And now they're forced to deal with the consequences, i.e. being called hypocrites.
I suspect that the argument you're referring to is more like "free speech does not imply that all deplatforming is unacceptable." Again, it is not the case that the only 2 consistent views are "all deplatforming is unacceptable" or "all deplatforming is acceptable."