Mach 5 refers to 5 times the speed of sound.
Irrelevant. First THEL doesn't actually work -- see the Hezbollah War for proof.
Second, the THEL has to paint the warhead for a sufficient amount of time before it heats up enough to explode. If it's moving faster, it'll have less exposure time before the laser is out of range.
Third, THEL is extremely vulnerable to countermeasures. So even if THEL worked as designed, and you were able to paint a hypersonic target, it would still be easy to cover an incoming warhead with a reflective coating.