No it does not matter, and I struggle to understand why anyone would think it does. There's no other context where every criticism of a private company gets countered with the argument that that private company isn't violating the constitution.

It matters in that we should not lobby congress to make reddit unban certain subreddits.

However, that does not mean we should not criticize reddit for not standing up for freedom of speech. You can't make your whole website based around "open and honest discussion" then ban subreddits where open and honest discussion was happening, but in ways you didn't like.

Reddit is not a US government entity, so the US constitutional right to free speech is generally not applicable to discussions of Reddit.

The exception is the case where a US government entity controls an account that is a mod of a subreddit, in which case they would be subject to the First Amendment when performing mod duties such as deleting user comments or banning users. This specific case would burden only that user and would not place any additional burden upon the non-government mods or upon Reddit itself or its admins.