I would not say that it's freedom of speech that makes it a DoS attack possibility. We've had freedom of speech for a while without having disinformation going rampant, because the distribution costs of bullshit were still about as large as those of valid facts, so even if creating the bullshit was cheaper than creating a rebuttal based on facts, the total cost of creation plus distribution was roughly equal.

It's the modern (social) media environment and its inherent ways of allocating attention that creates the extreme asymmetry in distribution costs, where it's now cheaper than ever to distribute attention-grabbing bullshit, but still pretty expensive to distribute comparatively-boring facts.