Yes, it's apparently true.
In an interview on The Jim and Sam Show, Roman Reigns talked about the Saudi Arabia 50-man Royal Rumble to take place in April and said that not only would it be the greatest rumble of all-time but also that it will air on the WWE Network as a special.
Reigns noted that he hoped he wasn't giving away any kind of spoiler when he dropped that information.
There have been rumours about participants but it should be noted that not all the superstars at the event will necessarily be in the rumble match itself. There is also scheduled to be an appearance by The Undertaker "in some form" and a rumoured one from Chris Jericho as well.