The FO job listed on the Mesa website states that you must meet requirements for an ATP or RATP, so no they're not requiring an ATP. If they pick anyone that isn't an ATP already is a different story. It also says they will cover the CTP course.

Requirements / Desired Skills and Experience

ATP 1500 Hours Total Time Requirements

• 1500 hours Total Flight Time
• 500 hours Cross Country Time
• 100 hours Total Night Time
• 50 hours Multi Engine Time, Fixed Wing
• 250 hours of Pilot in Command Time, Fixed wing
• 100 hours of Instrument Time
• Commercial Pilot Certificate with Multi Engine and Instrument ratings
• Current FAA First Class Medical Certificate
• Ability to demonstrate instrument proficiency
• ATP Written (CTP training can be provided to qualified applicants)