That's only true if they continue growing reliably after being deplatformed. There's no reason why that should necessarily be true.
I feel like there a tendency to believe that, since censorship is bad, censorship must also be completely ineffective, and we tell stories like "deplatforming is useless" to reinforce that. But I don't think that's a realistic approach. Censorship can and does work, in the right circumstances. Chinese millennials may know that something happened at Tiananmen Square in 1989, but they're fuzzy on the details and inclined to downplay it and keep quiet about it. That's censorship working as intended. Assuming your enemies are incompetent doesn't accomplish anything but a false sense of security.
I feel like there a tendency to believe that, since censorship is bad, censorship must also be completely ineffective, and we tell stories like "deplatforming is useless" to reinforce that. But I don't think that's a realistic approach. Censorship can and does work, in the right circumstances. Chinese millennials may know that something happened at Tiananmen Square in 1989, but they're fuzzy on the details and inclined to downplay it and keep quiet about it. That's censorship working as intended. Assuming your enemies are incompetent doesn't accomplish anything but a false sense of security.