Is that really true Ken (a genuine question)? Mach 5 is where hypersonic officially begins, and I've never heard the Phoenix being classed as a hypersonic weapon before.
From
Raytheon AIM-54 Phoenix
When an AIM-54A is launched, its Rocketdyne MK 47 or Aerojet MK 60 solid-fueled rocket motor (in an MXU-637/B propulsion section) propels it to a speed of Mach 4+.
From
https://en.wikipedia.org/wiki/AIM-54_Phoenix
The Phoenix has several guidance modes and achieves its longest range by using mid-course updates from the F-14A/B AWG-9 radar (APG-71 radar in the F-14D) as it climbs to cruise between 80,000 ft (24,000 m) and 100,000 ft (30,000 m) at close to Mach 5. Phoenix uses this high altitude to gain gravitational potential energy, which is later converted into kinetic energy as the missile dives at high velocity towards its target.