In very high profile cases it is not unusual for a judge to order a media blackout so as not to prejudice the case. Though this rarely happens in employment tribunals as they're not criminal court cases and rarely attract the attention of the media.It is actually pretty unusual for media (social and otherwise) to take much notice of the employment tribunals as they're usually simply last resorts where (former) employees of an organisation are seeking compensation and apology for poor/illegal treatment in the workplace.
The issue here is that the allegations and rebuttals attracted so much attention, in part because of the profile of Horner and RBR, that it would end up as "trial by media". The employment tribunal, if it gets that far, needs to consider the cases and evidence put forward by RBR and the former employee. Not a rampaging mob or salivating fans and journalists. Indeed probably two mobs, one with the pitchforks out for Horner and another supporting him.