The government is not enforcing a rule upon a private entity. It is enforcing a rule upon itself, specifically on the office of the president, and as embodied

The government is not enforcing a rule upon a private entity. It is enforcing a rule upon itself, specifically on the office of the president, and as embodied in the person currently occupying that office. Twitter, as a private entity, has the option to wipe out a user's block list, or to save it now and restore it later, and in the wake of this ruling, would be publicly justified in doing so for the @realdonaldtrump user. They would only be helping that particular user to obey the rules of their office. But they can also ignore it completely, and leave it to Trump to clear his own block list. The court might request Twitter's assistance in checking on whether its ruling is being honored, and they might choose to cooperate, or not, in the absence of any legal compulsion to do so. This isn't a Twitter thing. It's a government thing. The same thing has happened to government officials or agencies who blocked local gadflies and protesters from their Facebook pages.