Yes, it appears to be true! Spacetime Studios is working on a fourth title for its popular mobile MMO library, filing a trademark that has since been approved. The application for a U.S. federal trademark was submitted on April 5th and was given the thumbs up May 10th. As one of its categories is "electronic game software for cellular telephones," we can reasonably deduce that Arcane Legends will launch on mobile devices in the same manner of its older siblings.
Spacetime Studios told us that it will release an official announcement of the title later this week. The studio recently released the vampire-themed MMO Dark Legends on iOS and Android devices.