The linked essay, in describing Mill's argument, carefully makes the point that free speech beyond legality matters. It's not just what's protected from interference by the government, it's what's allowed by consensus, by social pressure, by available platforms, and by circumstance; these aspects can all have a practical effect of allowing or denying someone's freedom of expression.
Also, freedom of speech isn't a binary condition. As a practical matter, we're always balancing our attempt to maximize freedom against constraints required to continue to exist. Reddit, despite taking a radical freedom of speech line, won't allow child porn, but they also won't allow doxxing or incitement, which is their rationale for banning certain massively racist groups and not others. They've also woken up to the fact that certain very extreme subreddits threaten their long-term viability because people simply won't use Reddit while that association persists. Violentacrez was tolerated and encouraged right up until he became an existential liability.
Also, freedom of speech isn't a binary condition. As a practical matter, we're always balancing our attempt to maximize freedom against constraints required to continue to exist. Reddit, despite taking a radical freedom of speech line, won't allow child porn, but they also won't allow doxxing or incitement, which is their rationale for banning certain massively racist groups and not others. They've also woken up to the fact that certain very extreme subreddits threaten their long-term viability because people simply won't use Reddit while that association persists. Violentacrez was tolerated and encouraged right up until he became an existential liability.