If it was the government telling a platform to block certain speech, that would be a restriction in free speech.

The difference is that the ownership of HN has their OWN freedom to choose who they let post on here, and what they let be posted.

This is known as the 'Freedom of Association': The right to come together with other people to collectively express, promote, or pursue their common interest. This right also includes the right to accept or decline membership in the group for whatever reason they want. HN can choose to only allow whatever they choose on their site.

In the United States, this right is actually established (according to court rulings) by the same First Amendment that establishes the right to free speech.

The right to free speech does not require a private individual or company give you a platform to make your speech.